Return a tuple of None's instead of one None

The expected return value of _commit_port_binding is something that
can be unpacked into two values. This changes the condition where a
concurrent delete was detected to return a 2-tuple of None's
instead of a bare None to prevent an iterable error.

Closes-Bug: #1342880
Change-Id: I9d3a51eab33819ffda6a65a00c3b5d1006a3577e
Merge "Return a tuple of None's instead of one None"