Don't accept empty value as a valid one in SM

src/style_manager/model/Property.js

@@ -162,7 +162,7 @@ const Property = Backbone.Model.extend(
     getFullValue(val) {
       const fn = this.get('functionName');
       let value = isUndefined(val) ? this.get('value') : val;
-      const hasValue = !isUndefined(value);
+      const hasValue = !isUndefined(value) && value !== '';
 
       if (fn && hasValue) {
         value = `${fn}(${value})`;