fix: prevent NOTSET sentinel leaking as test ID in parametrize with callable ids

[rusthype]

Summary

Fixes #13235

When @pytest.mark.parametrize is used with an empty argvalues list and a callable ids parameter, two bugs occur:

1. The internal NOTSET sentinel string leaks into test output as test_foo[NOTSET]
2. The user's ids callable gets invoked with the NOTSET sentinel, causing an AttributeError or ValueError

@pytest.mark.parametrize("matrix", [], ids=lambda m: m.id)
def test_foo(matrix):
    pass

Before: test_foo[NOTSET] in output (string leaks) or crash

After: test_foo[matrix0] — clean, matches the auto-naming convention

Root Cause

In ParameterSet._for_parametrize(), when argvalues=[], a placeholder ParameterSet is created with id="NOTSET". This string was then yielded verbatim in _resolve_ids() (since id is not None), leaking the internal name. If the user provided a callable ids, it was also called on the NOTSET sentinel value before id="NOTSET" was set (in older versions) or was bypassed (current main), but the string still leaked.

Fix

src/_pytest/mark/structures.py — use id=None for the placeholder ParameterSet so standard ID generation runs:

# Before
ParameterSet(values=(NOTSET,) * len(argnames), marks=[mark], id="NOTSET")

# After
ParameterSet(values=(NOTSET,) * len(argnames), marks=[mark], id=None)

src/_pytest/python.py — guard _idval_from_function to skip calling the user's callable when val is NOTSET:

def _idval_from_function(self, val, argname, idx):
    if self.idfn is None:
        return None
    if val is NOTSET:      # ← new guard
        return None
    ...

This follows the existing pattern in _idval_from_value which already has a special case for NOTSET.

Test

Added TestMetafuncFunctional::test_parametrize_empty_argvalues_callable_ids in testing/python/metafunc.py verifying:

• No NOTSET appears in output
• The test is properly skipped (1 skipped)

Checklist

• Changelog entry added (changelog/13235.bugfix.rst)
• Test added covering the reported scenario
• Fix verified manually against the reproduction case from pytest.mark.parametrize ids with empty parameter #13235

[bluetech]

The NOTSET in this case is intentional.