When publish_errors is True, and using `messaging` notify driver,
produces a infinite loop that report sending notification error.
The reason is we always passing None to the content argument in
logger handlers (LoggingNotificationHandler, LoggingErrorNotificationHandler),
when pack the context object which expected as a dict, raise TypeError
exception, so result in infinite retry looping.
To match what document said, passing a empty dict rather than None. Also
modify unittests to make sure context is a empty dict.
Change-Id: Ic2c4c43f5bbafa8107ea370ba959da16cfa4a24c
Closes-bug: #1346466
fb8c431ad6