[Firebase AI] Handle a response with no Parts (#1319)

Author: a-maurice

firebaseai/src/ModelContent.cs

@@ -402,7 +402,7 @@ internal static ModelContent FromJson(Dictionary<string, object> jsonDict) {
       // If the role is missing, default to model since this is likely coming from the backend.
       jsonDict.ParseValue("role", defaultValue: "model"),
       // Unknown parts are converted to null, which we then want to filter out here
-      jsonDict.ParseObjectList("parts", PartFromJson, JsonParseOptions.ThrowEverything).Where(p => p is not null));
+      jsonDict.ParseObjectList("parts", PartFromJson)?.Where(p => p is not null));
   }
 
   private static InlineDataPart InlineDataPartFromJson(Dictionary<string, object> jsonDict,

firebaseai/testapp/Assets/Firebase/Sample/FirebaseAI/UIHandlerAutomated.cs

@@ -713,7 +713,7 @@ async Task TestGenerateImage(Backend backend) {
       );
 
       GenerateContentResponse response = await model.GenerateContentAsync(
-        ModelContent.Text("Can you give me a picture of a cartoon dog, and a couple of sentences about him?")
+        ModelContent.Text("Generate a picture of a cartoon dog, and a couple of sentences about him?")
       );
 
       Assert("Response missing candidates.", response.Candidates.Any());