bitbake: process: stop bb.process.communicate mixing bytes and str return types

bitbake: process: stop bb.process.communicate mixing bytes and str return types

Python3 regards b"" as False so it is not being converted to a string by
d0f904d407f57998419bd9c305ce53e5eaa36b24. This confusingly causes three
different potential types for each member of the returned tuple.

Let's just assume that everything that's not None is a bytes object and
convert it to a string.

(Bitbake rev: 0cf5589b7fb3582a6caca5014c4d8152347df545)

Signed-off-by: Mike Crowe <mac@mcrowe.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>