Handle empty list of values in parallel_map() (#4633)
Handle empty list of values in parallel_map() (#4633)

There was an edge case with parallel_map() if someone called it with an
empty list of values it would trigger the event pubsub machinery even if
though no work will actually be done. This commit adds a special case to
handle this (like what is done for a single value) to just return an
empty list fast if no work is being requested.

Fixes #3634

Co-authored-by: Kevin Krsulich <ffb4761cba839470133bee36aeb139f58d7dbaa9@krsulich.net>
QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py