Delete more empty pot files
Delete more empty pot files

Also delete empty pot files, there's no need to keep them around or send
them to the translation server.
For this, factor out the empty pot deletion from extract_messages_log
as separate function and use it everywhere.

This has been noticed in keystone where the zanata push shows:

"pushing source doc
[name=keystone_tempest_plugin/locale/keystone_tempest_plugin size=0] to
server"

Change-Id: I69ca0385408f8df52b20c1c93daae658003e9282

Tesora/tesora-project-config,dongwenjuan/project-config,openstack-infra/project-config,dongwenjuan/project-config,openstack-infra/project-config,Tesora/tesora-project-config