LSP: Avert a crash when an empty update is sent.
LSP: Avert a crash when an empty update is sent.

Summary:
Although we ask for full sync, some plugins appear to send an empty change set
in certain circumstances. Log and continue rather than crashing in that case.

Reviewers: salguarnieri, #core_team

Reviewed By: salguarnieri, #core_team

Subscribers: #core_team

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D1973

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