BUGFIX: Empty address fields with spaces getting treated as non-empty

[CaitlinCP]

Config parsing is duplicated between the CLI entrypoint and the Streamlit app (ui/logic.py)

There is a risk where the two paths normalize values inconsistently. Neither path handles explicitly blank YAML values.

SOLUTION:

• Include a Pydantic dependency
• Add a models.py at the package root with a GeocoderConfig Pydantic model
• Coerce empty strings to None in model validators
• Update the CLI entrypoint and parse_config in ui/logic.py to use GeocoderConfig
• Update Geocoder.__init__ to accept GeocoderConfig

ACCEPTANCE CRITERIA

• A YAML config with blank address_fields should no longer cause the geocoder to treat those fields as populated
• Both the CLI and app code paths produce equivalent config objects from the same YAML input