Fix Constant(None) field rejecting null input

[wingding12]

Summary

When Constant field is initialized with None as the constant value, loading a payload containing null would incorrectly raise:

ValidationError: {'sentinel': ['Field may not be null.']}

Root Cause

The issue was in the initialization order:

1. Constant.__init__ calls super().__init__(**kwargs)
2. Field.__init__ computes allow_none based on load_default (line 213)
3. At this point, load_default is still missing_ (not passed in kwargs)
4. So allow_none is computed as False
5. Only afterwards does Constant.__init__ set self.load_default = constant
6. But allow_none has already been incorrectly set

Fix

Pass load_default and dump_default to the parent constructor via kwargs.setdefault() so that allow_none is computed correctly when the constant is None.

Test Plan

Added test test_constant_none_allows_null_input that verifies:

• Loading {"sentinel": None} works without raising ValidationError
• Loading {"sentinel": "anything"} also works (constant always wins)
• Dumping also works correctly

Fixes #2868