help: Don't show empty Summary or Description sections

Signed-off-by: Shawn O. Pearce <sop@google.com>

subcmds/help.py

@@ -94,6 +94,8 @@ See 'repo help --all' for a complete list of recognized commands.
           body = getattr(cmd, bodyAttr)
         except AttributeError:
           return
+        if body == '' or body is None:
+          return
 
         self.nl()