gh-155877: Reject a NUL cell in curses.complexstr() from an iterable

[fedonman]

GH-156158 rejects a NUL in curses.complexstr() built from a str, but not from an iterable, so complexstr([complexchar('\0'), complexchar('B')]) from the issue still builds and addstr() drops every cell after the NUL. This adds the same check to curses_pack_cell(), which packs the items of an iterable, so a NUL item now raises ValueError: embedded null character too.

$ ./python -m test test_curses -u curses -m test_cell_null_char -v
test_cell_null_char (test.test_curses.TestCurses.test_cell_null_char) ... ok

$ ./python -m test test_curses -u curses
Total tests: run=189 skipped=4
Total test files: run=1/1
Result: SUCCESS

No NEWS entry: the cell API has not been released.

• Issue: curses cell text is silently truncated at an embedded NUL #155877