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.
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