Avoid cleaning doc when ZUUL_UUID is not set

d08c41f aborts the publish doc script whenever ZUUL_UUID is not set, but
we could potentially still run the cleanup postbuild task.  Without a
ZUUL_UUID that would delete everything from the publishing instance
which can delete a doc pending publishing.

Change-Id: I1acb509cfb395aa8087a765f4e021cb2b0a58f18