Don't call 'review' with empty ReviewInput

Apart from being unnecessary, every call to 'review' will remove the
ChangeOwner from the Attention-Set regardless of other states [1].
Not calling 'review' when not needed mitigates the effects of this
bug.
[1] https://bugs.chromium.org/p/gerrit/issues/detail?id=14941

Bug: Issue 14925
Change-Id: I314cfcb358ba08b1c1f5400dd7d6ec538342a99f