fix: Allow null value_column for map data overlay count metrics

[sentry]

This PR addresses a ValidationError occurring in the /api/v1/public/dashboards/{token}/charts/{chart_id}/map-data/ endpoint.

Root Cause:
Map charts using count aggregations do not require a specific value_column. However, the MapDataOverlayPayload schema (in ddpui/api/charts_api.py) declared value_column: str as a required, non-nullable field. When the frontend sent value_column: None for such charts, Pydantic validation failed.

Changes Made:

1. ddpui/api/charts_api.py: Modified MapDataOverlayPayload.value_column to be Optional[str] = None, allowing None as a valid value.
2. ddpui/api/charts_api.py: Removed value_column from the explicit if not all([...]) required-field validation check in get_map_data_overlay.
3. ddpui/api/public_api.py: Removed map_payload.value_column from the explicit if not all([...]) required-field validation check in get_public_map_data_overlay (both occurrences).

These changes ensure that map charts with count aggregations can successfully retrieve data without validation errors, as value_column is not relevant for these types of metrics.

Fixes DALGO-BACKEND-27Y

[sentry]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 58.66%. Comparing base (75cb6fd) to head (a9b9d64).

Files with missing lines	Patch %	Lines
ddpui/api/charts_api.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1383   +/-   ##
=======================================
  Coverage   58.66%   58.66%           
=======================================
  Files         132      132           
  Lines       15690    15690           
=======================================
  Hits         9205     9205           
  Misses       6485     6485           

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