Handle invocation with no available verbs or env vars

It might be useful when measuring performance to invoke colcon with an
entire class of extensions blocked. This change "handles" the case where
no verbs are available and minimizes the output when there are no verbs
or environment variables available.

colcon_core/command.py

@@ -232,14 +232,16 @@ def _parse_optional(self, arg_string):
                 return None
             return result
 
+    epilog = get_environment_variables_epilog(environment_variables_group_name)
+    if epilog:
+        epilog += '\n\n'
+    epilog += READTHEDOCS_MESSAGE
+
     # top level parser
     parser = CustomArgumentParser(
         prog=get_prog_name(),
         formatter_class=CustomFormatter,
-        epilog=(
-            get_environment_variables_epilog(
-                environment_variables_group_name
-            ) + '\n\n' + READTHEDOCS_MESSAGE))
+        epilog=epilog)
 
     # enable introspecting and intercepting all command line arguments
     parser = decorate_argument_parser(parser)
@@ -287,6 +289,8 @@ def get_environment_variables_epilog(group_name):
     """
     # list environment variables with descriptions
     entry_points = load_extension_points(group_name)
+    if not entry_points:
+        return ''
     env_vars = {
         env_var.name: env_var.description for env_var in entry_points.values()}
     epilog_lines = []
@@ -376,7 +380,6 @@ def create_subparser(parser, cmd_name, verb_extensions, *, attribute):
     :returns: The special action object
     """
     global colcon_logger
-    assert verb_extensions, 'No verb extensions'
 
     # list of available verbs with their descriptions
     verbs = []
@@ -387,9 +390,9 @@ def create_subparser(parser, cmd_name, verb_extensions, *, attribute):
     # add subparser with description of verb extensions
     subparser = parser.add_subparsers(
         title=f'{cmd_name} verbs',
-        description='\n'.join(verbs),
+        description='\n'.join(verbs) or None,
         dest=attribute,
-        help=f'call `{cmd_name} VERB -h` for specific help',
+        help=f'call `{cmd_name} VERB -h` for specific help' if verbs else None,
     )
     return subparser