fix(button): theme mixin not validating

PiperOrigin-RevId: 535046546

Author: asyncliz

button/lib/_elevated-button.scss

@@ -23,7 +23,7 @@
   );
 
   @each $token, $value in $tokens {
-    @if list.index($supported-tokens, $token) {
+    @if list.index($supported-tokens, $token) == null {
       @error 'Token `#{$token}` is not a supported token.';
     }
 

button/lib/_filled-button.scss

@@ -23,7 +23,7 @@
   );
 
   @each $token, $value in $tokens {
-    @if list.index($supported-tokens, $token) {
+    @if list.index($supported-tokens, $token) == null {
       @error 'Token `#{$token}` is not a supported token.';
     }
 

button/lib/_outlined-button.scss

@@ -26,7 +26,7 @@
   );
 
   @each $token, $value in $tokens {
-    @if list.index($supported-tokens, $token) {
+    @if list.index($supported-tokens, $token) == null {
       @error 'Token `#{$token}` is not a supported token.';
     }
 

button/lib/_text-button.scss

@@ -22,7 +22,7 @@
   );
 
   @each $token, $value in $tokens {
-    @if list.index($supported-tokens, $token) {
+    @if list.index($supported-tokens, $token) == null {
       @error 'Token `#{$token}` is not a supported token.';
     }
 

button/lib/_tonal-button.scss

@@ -22,7 +22,7 @@
   );
 
   @each $token, $value in $tokens {
-    @if list.index($supported-tokens, $token) {
+    @if list.index($supported-tokens, $token) == null {
       @error 'Token `#{$token}` is not a supported token.';
     }