Wrong return type for `get_metadata_call_function` methods.

[basfroman]

The next methods return type are wrong:

• async_substrate_interface.sync_substrate.SubstrateInterface.get_metadata_call_function
• async_substrate_interface.async_substrate.AsyncSubstrateInterface.get_metadata_call_function

These two methods should return scalecodec.types.GenericVariant type but not a Optional[list].
Linters are complains about types incompatibility.