- Functions which can return null always return a Python object

Minor change to check for and return None in method_function_returns_instance

Author: Tom Lofts <dev@loftx.co.uk>

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23116 57a11ea4-9604-0410-9ed3-97b8803252fd