Fix DictField HTML input returning empty dict for missing fields

[veeceey]

Summary

Fixes #6234

parse_html_dict always returned an empty MultiValueDict when no matching keys were found in the HTML form input, making it impossible to distinguish between:

• A field that was not specified in the form data
• A field that was specified but had no values

This caused DictField to treat missing fields as if they contained an empty dict, breaking required, default, and partial update logic for multipart/form-data requests.

Changes

• rest_framework/utils/html.py: Added a default parameter to parse_html_dict (consistent with how parse_html_list already works). Returns default when no matching keys are found in the input.
• rest_framework/fields.py: Updated DictField.get_value to pass default=empty so that missing fields are correctly treated as not provided. Also updated DictField.to_internal_value to pass default=data so inner MultiValueDict data is preserved when no dot-separated sub-keys are found.
• rest_framework/serializers.py: Updated Serializer.get_value to use the new default= parameter instead of or empty for consistency.
• tests/test_fields.py: Added 4 new test cases for DictField HTML form input covering:
  • Valid dict input via QueryDict with dot-separated keys
  • Missing field with a default value
  • Missing field that is not required (should be skipped)
  • Missing field that is required (should fail validation)

Test plan

• All existing DictField tests pass
• All 4 new test cases pass
• Full test_fields.py, test_serializer.py, and test_parsers.py suites pass (401 passed, 1 pre-existing failure unrelated to this change)
• Verify in a real Django project with multipart/form-data requests that DictField partial updates work correctly

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