CherryPick: do not append new branch to empty topic
This is a bugfix for I195774b7f594553018e13101a6b3ce904249eed6.
When there is no topic set, we should not set a topic on cherry-picking.
Users reported changes with no topic set, which were cherry-picked to have
the topic "null-${new-branch}" afterwards, which is annoying specially with
the new submit whole topic feature enabled as lots of unrelated changes
end up in the new topic.
Change-Id: Ice42d1e6cfbbf12dc3936bb2a0295df8b9e0907c
2 files changed
