xml_func.py: simplify(): Apply pass-through optimizations for _if statements with no condition (which means false). This faciliates automated testing after an _if statement has been added, because the put template provided as part of the automated test will only change for those datasources that actually have a condition entry for the _if statement, which greatly reduces the number of tests that need to be accepted. (Note that the path before the _if will still be included as an empty path if there are no other mappings to that table, because the _if statement does not surround it.)