fix: Test for empty string in addition to None Type in 'pagure.lib.git.commit_to_patch'
When 'test_commit_to_patch_empty_commit' runs, 'commit.tree.diff_to_tree(swap=True)' in 'pagure.lib.git.commit_to_patch' returns 'None' if pygit2 version is 1.8.0. With pygit2 version 1.14.x the return value is an empty string.