bnc#837302: Allow insertion of redlines with an empty author.

No idea why that was not allowed (since the initial import in 2000), we need
it for interoperability.

Conflicts:

	sw/qa/extras/ooxmlexport/ooxmlexport.cxx
	sw/source/core/doc/docredln.cxx

Change-Id: I475fa63f9a4a3219a2b898b6f7d81e0603d32c18
Reviewed-on: https://gerrit.libreoffice.org/6897
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>