build: don't flag noop mixins in theme mixin rule (#19558)

The `theme-mixin-api` rule flags any mixin that doesn't include a variable that extracts the config. This can be annoying for noop placeholder mixins since we have to declare a variable that won't be used. These changes add an exception for mixins that are empty or only contain comment nodes.

Author: crisbeto

src/material-experimental/column-resize/_column-resize.scss

@@ -97,13 +97,9 @@
   }
 }
 
-@mixin mat-column-resize-typography($config-or-theme) {
-  $config: mat-get-typography-config($config-or-theme);
-}
+@mixin mat-column-resize-typography($config-or-theme) {}
 
-@mixin mat-column-resize-density($config-or-theme) {
-  $density-scale: mat-get-density-config($config-or-theme);
-}
+@mixin mat-column-resize-density($config-or-theme) {}
 
 @mixin mat-column-resize-theme($theme-or-color-config) {
   $theme: _mat-legacy-get-theme($theme-or-color-config);

src/material-experimental/mdc-autocomplete/_autocomplete-theme.scss

@@ -1,22 +1,10 @@
 @import '../mdc-helpers/mdc-helpers';
 
-@mixin mat-mdc-autocomplete-color($config-or-theme) {
-  $config: mat-get-color-config($config-or-theme);
-  @include mat-using-mdc-theme($config) {
-    // TODO: implement MDC-based autocomplete.
-  }
-}
+@mixin mat-mdc-autocomplete-color($config-or-theme) {}
 
-@mixin mat-mdc-autocomplete-typography($config-or-theme) {
-  $config: mat-get-typography-config($config-or-theme);
-  @include mat-using-mdc-typography($config) {
-    // TODO: implement MDC-based autocomplete.
-  }
-}
+@mixin mat-mdc-autocomplete-typography($config-or-theme) {}
 
-@mixin mat-mdc-autocomplete-density($config-or-theme) {
-  $density-scale: mat-get-density-config($config-or-theme);
-}
+@mixin mat-mdc-autocomplete-density($config-or-theme) {}
 
 @mixin mat-mdc-autocomplete-theme($theme-or-color-config) {
   $theme: _mat-legacy-get-theme($theme-or-color-config);

src/material-experimental/mdc-button/_button-theme.scss

@@ -258,9 +258,7 @@ $mat-button-state-target: '.mdc-button__ripple';
   }
 }
 
-@mixin mat-mdc-fab-density($config-or-theme) {
-  $density-scale: mat-get-density-config($config-or-theme);
-}
+@mixin mat-mdc-fab-density($config-or-theme) {}
 
 @mixin mat-mdc-fab-theme($theme-or-color-config) {
   $theme: _mat-legacy-get-theme($theme-or-color-config);

src/material-experimental/mdc-card/_card-theme.scss

@@ -48,9 +48,7 @@
   }
 }
 
-@mixin mat-mdc-card-density($config-or-theme) {
-  $density-scale: mat-get-density-config($config-or-theme);
-}
+@mixin mat-mdc-card-density($config-or-theme) {}
 
 @mixin mat-mdc-card-theme($theme-or-color-config) {
   $theme: _mat-legacy-get-theme($theme-or-color-config);

src/material-experimental/mdc-input/_input-theme.scss

@@ -10,9 +10,7 @@
   @include mat-using-mdc-typography($config) {}
 }
 
-@mixin mat-mdc-input-density($config-or-theme) {
-  $density-scale: mat-get-density-config($config-or-theme);
-}
+@mixin mat-mdc-input-density($config-or-theme) {}
 
 @mixin mat-mdc-input-theme($theme-or-color-config) {
   $theme: _mat-legacy-get-theme($theme-or-color-config);

src/material-experimental/mdc-menu/_menu-theme.scss

@@ -52,9 +52,7 @@
   }
 }
 
-@mixin mat-mdc-menu-density($config-or-theme) {
-  $density-scale: mat-get-density-config($config-or-theme);
-}
+@mixin mat-mdc-menu-density($config-or-theme) {}
 
 @mixin mat-mdc-menu-theme($theme-or-color-config) {
   $theme: _mat-legacy-get-theme($theme-or-color-config);

src/material-experimental/mdc-progress-bar/_progress-bar-theme.scss

@@ -29,14 +29,9 @@
   }
 }
 
-@mixin mat-mdc-progress-bar-typography($config-or-theme) {
-  $config: mat-get-typography-config($config-or-theme)
-  // No typography for this component.
-}
+@mixin mat-mdc-progress-bar-typography($config-or-theme) {}
 
-@mixin mat-mdc-progress-bar-density($config-or-theme) {
-  $density-scale: mat-get-density-config($config-or-theme);
-}
+@mixin mat-mdc-progress-bar-density($config-or-theme) {}
 
 @mixin mat-mdc-progress-bar-theme($theme-or-color-config) {
   $theme: _mat-legacy-get-theme($theme-or-color-config);

src/material-experimental/mdc-sidenav/_sidenav-theme.scss

@@ -1,18 +1,8 @@
 @import '../mdc-helpers/mdc-helpers';
 
-@mixin mat-mdc-sidenav-color($config-or-theme) {
-  $config: mat-get-color-config($config-or-theme);
-  @include mat-using-mdc-theme($config) {
-    // TODO: implement MDC-based sidenav.
-  }
-}
+@mixin mat-mdc-sidenav-color($config-or-theme) {}
 
-@mixin mat-mdc-sidenav-typography($config-or-theme) {
-  $config: mat-get-typography-config($config-or-theme);
-  @include mat-using-mdc-typography($config) {
-    // TODO: implement MDC-based sidenav.
-  }
-}
+@mixin mat-mdc-sidenav-typography($config-or-theme) {}
 
 @mixin mat-mdc-sidenav-density($config-or-theme) {
   $density-scale: mat-get-density-config($config-or-theme);

src/material-experimental/mdc-slider/_slider-theme.scss

@@ -25,9 +25,7 @@
   }
 }
 
-@mixin mat-mdc-slider-density($config-or-theme) {
-  $density-scale: mat-get-density-config($config-or-theme);
-}
+@mixin mat-mdc-slider-density($config-or-theme) {}
 
 @mixin mat-mdc-slider-theme($theme-or-color-config) {
   $theme: _mat-legacy-get-theme($theme-or-color-config);

src/material-experimental/mdc-snackbar/_snackbar-theme.scss

@@ -1,16 +1,10 @@
 @import '../mdc-helpers/mdc-helpers';
 
-@mixin mat-mdc-snackbar-color($config-or-theme) {
-  $config: mat-get-color-config($config-or-theme);
-}
+@mixin mat-mdc-snackbar-color($config-or-theme) {}
 
-@mixin mat-mdc-snackbar-typography($config-or-theme) {
-  $config: mat-get-typography-config($config-or-theme);
-}
+@mixin mat-mdc-snackbar-typography($config-or-theme) {}
 
-@mixin mat-mdc-snackbar-density($config-or-theme) {
-  $density-scale: mat-get-density-config($config-or-theme);
-}
+@mixin mat-mdc-snackbar-density($config-or-theme) {}
 
 @mixin mat-mdc-snackbar-theme($theme-or-color-config) {
   $theme: _mat-legacy-get-theme($theme-or-color-config);