Fix Internal Server error in coverage metadata request

[mmerdes]

Currently, the ohsome-dashboard frontend issues request like so:

https://api.quality.ohsome.org/v1/metadata/indicators/mapping-saturation/coverage?inverse=true

this leads to a crash in the oqapi backend.
This behavior can also be demonstrated via swagger.

The following erros occur in the log file:

fastapi.exceptions.ResponseValidationError: 2 validation errors:
  {'type': 'int_type', 'loc': ('response', 'features', 0, 'id', 'int'), 'msg': 'Input should be a valid integer', 'input': {"coordinates": [], "type": "Polygon"}, 'url': 'https://errors.pydantic.dev/2.8/v/int_type'}
  {'type': 'string_type', 'loc': ('response', 'features', 0, 'id', 'str'), 'msg': 'Input should be a valid string', 'input': {"coordinates": [], "type": "Polygon"}, 'url': 'https://errors.pydantic.dev/2.8/v/string_type'}