LibImageDecoderClient: Decoded images with 0 frames are not successful

Previously you could pass anything (e.g a text file) to ImageDecoder and
it would "succeed" in decoding it and give you back a 0-frame result.

Let's consider that state a failure instead.

Userland/Libraries/LibImageDecoderClient/Client.cpp

@@ -51,6 +51,9 @@ Optional<DecodedImage> Client::decode_image(const ByteBuffer& encoded_data)
 
     auto& response = response_or_error.value();
 
+    if (response.bitmaps().is_empty())
+        return {};
+
     DecodedImage image;
     image.is_animated = response.is_animated();
     image.loop_count = response.loop_count();