Forbid empty custom property values

sass-spec: sass/sass-spec#1385
Fixes #2655

Author: glebm

src/expand.cpp

@@ -253,7 +253,13 @@ namespace Sass {
     if (value) value = value->perform(&eval);
     Block_Obj bb = ab ? operator()(ab) : NULL;
     if (!bb) {
-      if (!value || (value->is_invisible() && !d->is_important())) return 0;
+      if (!value || (value->is_invisible() && !d->is_important())) {
+        if (d->is_custom_property()) {
+          error("Custom property values may not be empty.", d->value()->pstate(), traces);
+        } else {
+          return nullptr;
+        }
+      }
     }
     Declaration* decl = SASS_MEMORY_NEW(Declaration,
                                         d->pstate(),

src/parser.cpp

@@ -1859,7 +1859,7 @@ namespace Sass {
       css_error("Invalid CSS", " after ", message);
     }
 
-    if (schema->empty()) return {};
+    if (schema->empty()) error("Custom property values may not be empty.");
     return schema.detach();
   }