compose [nfc]: Extract isTopicEmpty helper

This will later (in another PR) be useful for checking emptiness
when realmEmptyTopicDisplayName is given.

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/widgets/compose_box.dart

@@ -160,10 +160,12 @@ class ComposeTopicController extends ComposeController<TopicValidationError> {
     return trimmed.isEmpty ? kNoTopicTopic : trimmed;
   }
 
+  bool get isTopicEmpty => textNormalized == kNoTopicTopic;
+
   @override
   List<TopicValidationError> _computeValidationErrors() {
     return [
-      if (mandatory && textNormalized == kNoTopicTopic)
+      if (mandatory && isTopicEmpty)
         TopicValidationError.mandatoryButEmpty,
 
       if (