Update AI models default and optional values

To have stronger code

scribe/lib/scribe/ai/data/model/ai_api_response.dart

@@ -14,7 +14,7 @@ class AIApiResponse {
 
   Map<String, dynamic> toJson() => _$AIApiResponseToJson(this);
 
-  String? get content => choices.isNotEmpty ? choices[0].message.content : null;
+  String get content => choices.isNotEmpty ? choices[0].message.content : '';
 }
 
 @JsonSerializable()

scribe/lib/scribe/ai/data/repository/ai_repository_impl.dart

@@ -9,6 +9,6 @@ class AIScribeRepositoryImpl implements AIScribeRepository {
 
   @override
   Future<AIResponse> generateText(String prompt) async {
-    return await _dataSource.request(prompt);
+    return _dataSource.request(prompt);
   }
 }

scribe/lib/scribe/ai/domain/model/ai_response.dart

@@ -1,5 +1,5 @@
 class AIResponse {
-  final String? result;
+  final String result;
 
   const AIResponse({required this.result});
 

scribe/lib/scribe/ai/presentation/model/ai_action.dart

@@ -24,6 +24,6 @@ class CustomPromptAction extends AIAction {
 
   @override
   String getLabel(BuildContext context) {
-    return ScribeLocalizations.of(context)!.customPromptAction;
+    return ScribeLocalizations.of(context)?.customPromptAction ?? 'Help me write';
   }
 }