email_mirror: Clean up TestStreamEmailMessagesEmptyBody.
email_mirror: Clean up TestStreamEmailMessagesEmptyBody.

These tests have some code and comments that only used to apply when
these empty body scenarios used to raise the regular
ZulipEmailForwardError - now they raise ZulipEmailForwardUserError.
We adapt the tests to this fact and test by mocking logging.warning and
making sure it gets called with the intended warning message. This is
also needed to cover the ZulipEmailForwardUserError case with tests to
get to 100% coverage of email_mirror.py.

timabbott/zulip,hackerkid/zulip,rishig/zulip,showell/zulip,rht/zulip,brainwane/zulip,shubhamdhama/zulip,rishig/zulip,hackerkid/zulip,zulip/zulip,andersk/zulip,brainwane/zulip,punchagan/zulip,synicalsyntax/zulip,brainwane/zulip,showell/zulip,rishig/zulip,tommyip/zulip,hackerkid/zulip,timabbott/zulip,zulip/zulip,hackerkid/zulip,kou/zulip,eeshangarg/zulip,rishig/zulip,punchagan/zulip,showell/zulip,andersk/zulip,eeshangarg/zulip,tommyip/zulip,hackerkid/zulip,kou/zulip,tommyip/zulip,rht/zulip,showell/zulip,rishig/zulip,brainwane/zulip,punchagan/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,timabbott/zulip,kou/zulip,shubhamdhama/zulip,shubhamdhama/zulip,kou/zulip,brainwane/zulip,rht/zulip,brainwane/zulip,eeshangarg/zulip,zulip/zulip,synicalsyntax/zulip,timabbott/zulip,andersk/zulip,showell/zulip,hackerkid/zulip,kou/zulip,tommyip/zulip,timabbott/zulip,kou/zulip,synicalsyntax/zulip,synicalsyntax/zulip,punchagan/zulip,rishig/zulip,synicalsyntax/zulip,synicalsyntax/zulip,tommyip/zulip,rishig/zulip,punchagan/zulip,hackerkid/zulip,shubhamdhama/zulip,andersk/zulip,timabbott/zulip,zulip/zulip,eeshangarg/zulip,showell/zulip,eeshangarg/zulip,shubhamdhama/zulip,rht/zulip,synicalsyntax/zulip,tommyip/zulip,punchagan/zulip,andersk/zulip,eeshangarg/zulip,tommyip/zulip,zulip/zulip,brainwane/zulip,zulip/zulip,shubhamdhama/zulip,shubhamdhama/zulip,showell/zulip,rht/zulip,punchagan/zulip,andersk/zulip,kou/zulip,timabbott/zulip,rht/zulip,zulip/zulip