Panic is produced when adding media items to an album

[pacoorozco]

Describe the bug
The bug was reported on gphotosuploader/gphotos-uploader-cli#262. In some cases (I don't know when) Google Photos response include no MediaItem. The code panics when it tries to access to a nil MediaItem.

To Reproduce
When creating a media item the lines:

       mediaItemsResult := make([]MediaItem, len(result.NewMediaItemResults))
	for i, res := range result.NewMediaItemResults {
		m := res.MediaItem
		mediaItemsResult[i] = r.convertPhotosLibraryMediaItemToMediaItem(m)
	}

If result.NewMediaItemResults has some MediaItem pointing to nil, the code fails when it tries to access to it.

Expected behavior
It should not panic.

Additional context
The Google Photos API documentation is not saying anything about MediaItem pointing to nil on successful responses..

[pacoorozco]

Google Photos API could return a empty MediaItem when an error has occurred. In that case the response has a Status addressing the problem.

It's documented here

In that case, we are going to skip this returned value.

[pacoorozco]

Merged on master, v2.1.2