Fix sending error when subject parameter is unset.
Fix sending error when subject parameter is unset.

If we're sending a reply to a thread and there's no subject given, use the
thread's subject; otherwise use the empty string. Fixes TypeError in gmail
sending code.

Eagles2F/sync-engine,ErinCall/sync-engine,PriviPK/privipk-sync-engine,gale320/sync-engine,Eagles2F/sync-engine,nylas/sync-engine,jobscore/sync-engine,Eagles2F/sync-engine,wakermahmud/sync-engine,gale320/sync-engine,wakermahmud/sync-engine,PriviPK/privipk-sync-engine,EthanBlackburn/sync-engine,closeio/nylas,nylas/sync-engine,ErinCall/sync-engine,rmasters/inbox,ErinCall/sync-engine,EthanBlackburn/sync-engine,PriviPK/privipk-sync-engine,Eagles2F/sync-engine,nylas/sync-engine,jobscore/sync-engine,Eagles2F/sync-engine,gale320/sync-engine,wakermahmud/sync-engine,wakermahmud/sync-engine,gale320/sync-engine,ErinCall/sync-engine,EthanBlackburn/sync-engine,PriviPK/privipk-sync-engine,nylas/sync-engine,jobscore/sync-engine,closeio/nylas,rmasters/inbox,rmasters/inbox,closeio/nylas,EthanBlackburn/sync-engine,wakermahmud/sync-engine,ErinCall/sync-engine,PriviPK/privipk-sync-engine,closeio/nylas,jobscore/sync-engine,EthanBlackburn/sync-engine,gale320/sync-engine,rmasters/inbox