FIX If the help of an argument is None, don't fail

This is a fix for a regression introduced in click-contrib#134. It's possible that the
argument has a `help` attribute, but the value of it is `None`. We then hit a
type error where we pass `None` to `re.sub`.

sphinx_click/ext.py

@@ -218,9 +218,10 @@ def _format_argument(arg: click.Argument) -> ty.Generator[str, None, None]:
         )
     )
     # Subclasses of click.Argument may add a `help` attribute (like typer.main.TyperArgument)
-    if hasattr(arg, 'help'):
+    help = getattr(arg, 'help', None)
+    if help:
         yield ''
-        help_string = ANSI_ESC_SEQ_RE.sub('', getattr(arg, 'help'))
+        help_string = ANSI_ESC_SEQ_RE.sub('', help)
         for line in _format_help(help_string):
             yield _indent(line)
 

tests/test_formatter.py

@@ -185,6 +185,7 @@ def __init__(self, *args, help=None, **kwargs):
         @click.command()
         @click.option('--option', help='A sample option')
         @click.argument('ARG', help='A sample argument', cls=CustomArgument)
+        @click.argument('ARG_NO_HELP', cls=CustomArgument)
         def foobar(bar):
             """A sample command."""
             pass
@@ -200,7 +201,7 @@ def foobar(bar):
         .. program:: foobar
         .. code-block:: shell
 
-            foobar [OPTIONS] ARG
+            foobar [OPTIONS] ARG ARG_NO_HELP
 
         .. rubric:: Options
 
@@ -216,6 +217,10 @@ def foobar(bar):
 
             A sample argument
 
+
+        .. option:: ARG_NO_HELP
+
+            Required argument
         """
             ).lstrip(),
             '\n'.join(output),