Don't send empty label updates
Don't send empty label updates

Summary:
Whenever we changed the labels on a message we would unconditionally issue two
commands, +X-GM-LABELS and -X-GM-LABELS even if we had no labels to pass to
one of them. Now we only issue each command if we actually have labels to pass.

Test Plan: Run tests, run locally

Reviewers: khamidou, drew, bengotow, spang

Reviewed By: spang

Subscribers: jenkins

Differential Revision: https://phab.nylas.com/D3140

jobscore/sync-engine,closeio/nylas,jobscore/sync-engine,closeio/nylas,nylas/sync-engine,closeio/nylas,nylas/sync-engine,closeio/nylas,nylas/sync-engine,nylas/sync-engine,jobscore/sync-engine,jobscore/sync-engine