Added null check to fix NullReferenceException in api call.

[nphmuller]

When the reponse body from the Spotify servers would be empty, there could be a NullReferenceException in the Web client. This, for example, did at least occur in every call to SaveAlbums.

[JohnnyCrazy]

Thanks for catching this, I totally forgot about empty bodies when adding the Header-stuff
While this technically fixes the problem, it doesn't provide the headers (which should always be provided) if the body is empty.

Another approach would be adding a empty "{}" if the response is empty. This way, an ErrorResponse will be created and headers can be set.

I would exchange this line: SpotifyWebClient.cs#L121 with:

//Added empty object if data is empty --> Object will be created --> Headers can be retrieved
 response = new Tuple<ResponseInfo, string>(data.Item1, data.Item2.Length > 0 ? _encoding.GetString(data.Item2) : "{}"); 

Thoughts?

[nphmuller]

Totally agree. It's much better to fix the root cause.

I also implemented the fix on the Download methods, since it is theoretically possible for the bug to also occur there.

[JohnnyCrazy]

Perfect, we may consider adding a check to HasError() checking the status-header in the future, but since every error comes with an error-body, we should be good to go