Fix processing additionalProperties when bool-type

[Matthijsy]

I am just starting to use rigging, in combination with the fabric-rti-mcp. Using this crashed the _process_field since it returns a schema like this:

{'additionalProperties': True, 'type': 'object'}

This results in trying to process True as a dict. This PR first checks if the additionalProperties value actually is a dict before processing.

Please let me know if there is any feedback.