BUG: ensure text padding ufuncs handle stringdtype nan-like nulls

[ngoldbaum]

Currently these ufuncs fail for arrays containing null strings with the following error:

Traceback (most recent call last):
  File "/Users/goldbaum/Documents/numpy/../numpy-experiments/test.py", line 5, in <module>
    print(np.strings.center(arr, 3))
          ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/goldbaum/Documents/numpy/build-install/usr/local/lib/python3.11/site-packages/numpy/_core/strings.py", line 629, in center
    width = np.maximum(str_len(a), width)
                       ^^^^^^^^^^
ValueError: The length of a null string is undefined

The use of str_len in np.strings is only necessary for fixed-width strings, so I moved the python width calculations in the np.strings wrappers after the check for stringdtype in each function and added width checking in the C++ ufunc loops.

I also modified the tests to register the text-padding ufuncs as passing through nan-like null strings.

[ngoldbaum]

I initially marked this as a backport candidate but that was wrong, these ufuncs won't be in numpy 2.0.

[mhvk]

Guess we should have caught that the maximum width was really not necessary for StringDType. Anyway, solution looks all good!

[lysnikolaou]

LGTM!

[ngoldbaum]

Thanks for looking this over!