Summary:
Changelog: Fix [TypeError: Network request failed] on file upload
# Problem
In https://github.com/facebook/react-native/commit/31980094107ed37f8de70972dbcc319cc9a26339 I made method `loadImageForURL` blocking and nil returning since it was no longer cancellable.
However inside `[RCTNetworkTask validateRequestToken]` was a logic counting on request token being non nil.
This diff removes this check and adds `nullable` to `[RCTImageURLLoader loadImageForURL]` making it explicit.
Reviewed By: PeteTheHeat
Differential Revision: D22767174
fbshipit-source-id: 04d5562e381912233b9c14e8156cbf145288f063
Samuel Susla
ffc90c7f92