Compare to inspect.Parameter.empty by identity in docments

[kevin9327]

docments and DocmentTbl compare parameter defaults and annotations to inspect.Parameter.empty with == / !=. When a default's == doesn't return a plain bool, such as a numpy array, that comparison raises:

import numpy as np
from fastcore.docments import DocmentTbl
def f(a=np.array([1, 2])): ...
DocmentTbl(f)
# ValueError: The truth value of an array with more than one element is ambiguous.

This is the root cause of AnswerDotAI/nbdev#1322 (nbdev_readme / show_doc crashing on such a function). The traceback there points at the old nbdev copy of this code, but the same comparisons live in fastcore.docments now.

empty is a sentinel, so the fix compares by identity (is / is not) at every place in 04_docments.ipynb. fastcore/docments.py was regenerated with nbdev-export.

Test: a new cell uses a default whose __eq__/__ne__ raise, and checks docments(..., full=True) and DocmentTbl. It fails with ValueError before the change and passes after. nbdev-test nbs/04_docments.ipynb passes. The full nbdev-test run gives the same notebook failures with and without this change on my machine (network/optional deps: 02, 03, 03a, 03b, 05, 12, 13).

I used an AI assistant to help write this change; I reproduced the bug, checked the fix and ran the tests myself.