fix breaking arg parsing when plugin not in use

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "certbot-onion"
-version = "0.1.5"
+version = "0.1.6"
 description = "Certbot authenticator plugin for the onion-csr-01 challenge"
 authors = [
     {name = "Q Misell", email = "[email protected]"}

src/certbot_onion/__init__.py

@@ -48,10 +48,12 @@ def __init__(self, *args, **kwargs):
 
     @classmethod
     def add_parser_arguments(cls, add: typing.Callable[..., None]) -> None:
-        add("hs-dir", help="Path to a Tor hidden service directory", nargs="+", required=True)
+        add("hs-dir", help="Path to a Tor hidden service directory", nargs="+", required=False)
 
     def prepare(self) -> None:
-        for hs_dir in self.conf("hs-dir"):
+        hs_dirs = self.conf("hs-dir")
+        hs_dirs = hs_dirs if hs_dirs is not None else []
+        for hs_dir in hs_dirs:
             if not os.path.isdir(hs_dir):
                 raise certbot.errors.PluginError(f"Hidden service directory path {hs_dir} is not a directory")