Use "must not" instead of "cannot" wording

ext/mbstring: Use standard wording for ValueError

Fix GH-11567: mb_str_pad causes access violation

When not providing a pad string, *and* not having other defaulted
arguments, the function would crash on a NULL pad zend_string*.
Des

Fix GH-11567: mb_str_pad causes access violation

When not providing a pad string, *and* not having other defaulted
arguments, the function would crash on a NULL pad zend_string*.
Despite testing with an empty pad string, the issue wasn't found because
when using named arguments the pad string *is* filled in.

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[RFC] Implement mb_str_pad() (#11284)

Closes GH-10203.