Allow python_exec kwarg to be None

processutils.execute() is documented to take a python_exec kwarg and
default to using sys.executable if that argument isn't set.  It is
convenient (and more intuitive) for this behavior to also occur if
the argument is present with value None.

Closes-bug: #1962603
Related-bug: #1962581
Change-Id: I2e1f187feaf4bf9fbfaf04fce78efa0cba49fc07