Explicitly return None in various Device methods.
Explicitly return None in various Device methods.

Previously, None was returned implicitly.

These changes are all precisely semantics preserving. In some cases the
result was treated as a boolean, in some cases it was explicitly compared
to None. The new code copies the old code.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

pyudev/pyudev