fix(firebase_ai): add validation for PromptFeedback parsing and handle empty cases (#17753)

Author: SelaseKay

packages/firebase_ai/firebase_ai/lib/src/api.dart

@@ -1372,6 +1372,12 @@ Candidate _parseCandidate(Object? jsonObject) {
 }
 
 PromptFeedback _parsePromptFeedback(Object jsonObject) {
+  if (jsonObject is! Map) {
+    throw unhandledFormat('PromptFeedback', jsonObject);
+  }
+  if (jsonObject.isEmpty) {
+    return PromptFeedback(null, null, []);
+  }
   return switch (jsonObject) {
     {
       'safetyRatings': final List<Object?> safetyRatings,

packages/firebase_ai/firebase_ai/test/response_parsing_test.dart

@@ -67,6 +67,32 @@ void main() {
       );
     });
 
+    test('with empty promptFeedback', () {
+      const response = '''
+{
+  "candidates": [
+    {
+      "content": {
+        "parts": [
+          {
+            "text": "Mountain View, California, United States"
+          }
+        ],
+        "role": "model"
+      },
+      "index": 0
+    }
+  ],
+  "promptFeedback": {}
+}
+''';
+      final decoded = jsonDecode(response) as Object;
+      expect(
+        VertexSerialization().parseGenerateContentResponse(decoded),
+        isA<GenerateContentResponse>(),
+      );
+    });
+
     test('with a blocked prompt', () {
       const response = '''
 {