fix `rmCommonWS()` helper to not crash when the template string fed to it has no content to (calculate the) unindent.
fix `rmCommonWS()` helper to not crash when the template string fed to it has no content to (calculate the) unindent.

GerHobbelt/jison,GerHobbelt/jison,GerHobbelt/jison,GerHobbelt/jison