histedit: drop fallback to empty string from rendertemplate() AFAICT, `cmdutil.rendertemplate()` always returns bytes (never e.g. `None`), so we don't need to fall back to empty (byte-)string. The fallback has been there since the code was added in 11c076786d56 (histedit: add templating support to histedit's rule file generation, 2019-01-29). Differential Revision: https://phab.mercurial-scm.org/D9244