test(radio-group): use correct validator check

Author: thetaPC

core/src/components/radio-group/test/validation/index.html

@@ -106,7 +106,7 @@ <h2>Optional Field (No Validation)</h2>
       // Validation functions
       const validators = {
         'fruits-radio-group': (value) => {
-          return value !== undefined;
+          return value !== '' && value !== undefined;
         },
         'optional-radio-group': () => true, // Always valid
       };