Update specs to support empty custom properties (#2058)

Author: nex3

spec/css/custom_properties/empty.hrx

@@ -0,0 +1,21 @@
+<===> literal/input.scss
+.empty {
+  --property:;
+}
+
+<===> literal/output.css
+.empty {
+  --property:;
+}
+
+<===>
+================================================================================
+<===> interpolation/input.scss
+.empty {
+  --property:#{""};
+}
+
+<===> interpolation/output.css
+.empty {
+  --property:;
+}

spec/css/custom_properties/error.hrx

@@ -1,35 +1,3 @@
-<===> empty/input.scss
-// CSS requires at least one token in a custom property.
-.empty {
-  --property:;
-}
-
-<===> empty/error
-Error: Expected token.
-  ,
-3 |   --property:;
-  |              ^
-  '
-  input.scss 3:14  root stylesheet
-
-<===>
-================================================================================
-<===> empty_interpolation/input.scss
-// CSS requires at least one token in a custom property.
-.empty {
-  --property:#{""};
-}
-
-<===> empty_interpolation/error
-Error: Custom property values may not be empty.
-  ,
-3 |   --property:#{""};
-  |              ^^^^^
-  '
-  input.scss 3:14  root stylesheet
-
-<===>
-================================================================================
 <===> nested/input.sass
 .no-nesting
   --foo: bar