gh-155499: Allow zero defaults with wide curses borders

[ahmedtaha100]

@fedonman

Fixes #155499.

border() and box() document 0 as the request for the default character. The wide-character path treated it as a narrow character, so mixing it with a string raised TypeError.

Treat zero like an omitted argument when calling wborder_set(), while continuing to reject nonzero integer or byte characters mixed with wide characters. The tests cover the reported border() call and both box() argument orders.

Tests:

• ./python -m test -v -u curses test_curses (168 passed)
• make patchcheck
• git diff --check

• Issue: curses border() and box() no longer accept 0 with a str #155499

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[picnixz]

Sorry but #155521 was opened first.