Pass a dict, instead of None, to modules if a None config is specified in the homeserver config (#9229)

If a Synapse module's config block were empty in YAML, thus being translated to a `Nonetype` in Python, then some modules could fail as that None ends up getting passed to their `parse_config` method. Modules are expected to accept a `dict` instead.

This PR ensures that if the user does end up specifying an empty config block (such as what [the default oidc config in the sample config](5310808d3b/docs/sample_config.yaml (L1816-L1845)) states) then `None` is not passed to the module. An empty dict is passed instead.

This code assumes that no existing modules are relying on receiving a `None` config block, but I'd really hope that they aren't.

changelog.d/9229.bugfix

@@ -0,0 +1 @@
+Fix a bug where `None` was passed to Synapse modules instead of an empty dictionary if an empty module `config` block was provided in the homeserver config.

synapse/util/module_loader.py

@@ -49,7 +49,8 @@ def load_module(provider: dict, config_path: Iterable[str]) -> Tuple[Type, Any]:
     module = importlib.import_module(module)
     provider_class = getattr(module, clz)
 
-    module_config = provider.get("config")
+    # Load the module config. If None, pass an empty dictionary instead
+    module_config = provider.get("config") or {}
     try:
         provider_config = provider_class.parse_config(module_config)
     except jsonschema.ValidationError as e: