Don't fail in event import if message From header is missing.
Don't fail in event import if message From header is missing.

Fixes T2326.

ErinCall/sync-engine,ErinCall/sync-engine,gale320/sync-engine,gale320/sync-engine,Eagles2F/sync-engine,closeio/nylas,Eagles2F/sync-engine,ErinCall/sync-engine,jobscore/sync-engine,nylas/sync-engine,nylas/sync-engine,Eagles2F/sync-engine,ErinCall/sync-engine,Eagles2F/sync-engine,gale320/sync-engine,nylas/sync-engine,closeio/nylas,closeio/nylas,jobscore/sync-engine,jobscore/sync-engine,nylas/sync-engine,ErinCall/sync-engine,gale320/sync-engine,gale320/sync-engine,closeio/nylas,Eagles2F/sync-engine,jobscore/sync-engine