Support STAC items without bbox

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STAC items are not required to have a bbox, the field is prohibited for items having no geometry.

This PR allows to validate an API that returs such items.

Otherwise you face this error:

      Traceback (most recent call last):
        File "/home/vprivat/projects/env/lib/python3.12/site-packages/behave/model.py", line 1909, in run
          match.run(runner.context)
        File "/home/vprivat/projects/env/lib/python3.12/site-packages/behave/matchers.py", line 104, in run
          self.func(context, *args, **kwargs)
        File "features/steps/stac.py", line 158, in step_check_stac_api
          (warnings, errors) = validate_api(
                               ^^^^^^^^^^^^^
        File "/home/vprivat/projects/env/lib/python3.12/site-packages/stac_api_validator/validations.py", line 673, in validate_api
          validate_item_search(
        File "/home/vprivat/projects/env/lib/python3.12/site-packages/stac_api_validator/validations.py", line 1427, in validate_item_search
          validate_item_search_ids_does_not_override_all_other_params(
        File "/home/vprivat/projects/env/lib/python3.12/site-packages/stac_api_validator/validations.py", line 3343, in validate_item_search_ids_does_not_override_all_other_params
          _validate_search_ids_with_ids_no_override(
        File "/home/vprivat/projects/env/lib/python3.12/site-packages/stac_api_validator/validations.py", line 3238, in _validate_search_ids_with_ids_no_override
          bbox = item["bbox"]
                 ~~~~^^^^^^^^
      KeyError: 'bbox'

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Thank you :)