Hide verb block from help if there are no verbs

Rather than show an empty verb block when there are no verbs, just hide
the block from the help text entirely.

Follow-up to 6cf24ea

colcon_core/command.py

@@ -428,7 +428,9 @@ def create_subparser(parser, cmd_name, verb_extensions, *, attribute):
         title=f'{cmd_name} verbs',
         description='\n'.join(verbs) or None,
         dest=attribute,
-        help=f'call `{cmd_name} VERB -h` for specific help' if verbs else None,
+        help=(
+            f'call `{cmd_name} VERB -h` for specific help' if
+            verbs else argparse.SUPPRESS),
     )
     return subparser