Pass a dict, instead of None, to modules if a None config is specified in the...

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](https://github.com/matrix-org/synapse/blob/5310808d3bebd17275355ecd474bc013e8c7462d/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.