Fix NPE sending mail on servers without Outbox folders

* stopPing (in SyncManager) assumes that every mailbox has a serverId
  but this is not the case on some servers, in which case we hit an
  NPE during a check for the account mailbox
* Check for a null serverId when testing for the account mailbox

Bug: 2606385
Change-Id: Idfa8abd8ef9e2c0a2ac01d0b168a21c934f6fdf3
1 file changed