`fields.Constant` rejects `None` constant values during load

[T90REAL]

When that constant is None, loading any payload that contains null triggers ValidationError: Field may not be null. The stems from the initialization order: Field.__init__ sets allow_none based on the (yet-unknown) load_default, and only afterwards Constant.__init__ assigns the actual constant/load_default.

from marshmallow import Schema, fields

class DemoSchema(Schema):
    # Should always load/dump None, independent of input
    sentinel = fields.Constant(None)

schema = DemoSchema()

assert schema.dump({"sentinel": "anything"})["sentinel"] is None

schema.load({"sentinel": None})

Traceback (most recent call last):
  File "/data/src/test.py", line 11, in <module>
    schema.load({"sentinel": None})
  File "/home/hdd/miniconda3/envs/py312/lib/python3.12/site-packages/marshmallow/schema.py", line 730, in load
    return self._do_load(
           ^^^^^^^^^^^^^^
  File "/home/hdd/miniconda3/envs/py312/lib/python3.12/site-packages/marshmallow/schema.py", line 938, in _do_load
    raise exc
marshmallow.exceptions.ValidationError: {'sentinel': ['Field may not be null.']}

Note: This issue was identified by an automated testing tool for academic research and manually verified. If you have any concerns about this type of reporting, please let me know, and I will adjust my workflow accordingly.

[sloria]

Thanks for reporting! I agree the current behavior is unexpected. PRs welcome!