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

hgext/histedit.py

@@ -525,12 +525,9 b' class histeditaction(object):'
         """
         ctx = self.repo[self.node]
         ui = self.repo.ui
-        summary = (
-            cmdutil.rendertemplate(
+        summary = cmdutil.rendertemplate(
                 ctx, ui.config(b'histedit', b'summary-template')
             )
-            or b''
-        )
         summary = summary.splitlines()[0]
         line = b'%s %s %s' % (self.verb, ctx, summary)
         # trim to 75 columns by default so it's not stupidly wide in my editor
@@ -1152,12 +1149,9 b' class histeditrule(object):'
 
     @util.propertycache
     def desc(self):
-        summary = (
-            cmdutil.rendertemplate(
+        summary = cmdutil.rendertemplate(
                 self.ctx, self.ui.config(b'histedit', b'summary-template')
             )
-            or b''
-        )
         if summary:
             return summary
         # This is split off from the prefix property so that we can