gh-155877: Fix a NUL in a curses character cell (GH-156158)

setcchar() takes a NUL-terminated string, so a NUL sharing a cell dropped
the rest of the cell, and an empty cell read back as complexchar('').

Reject a NUL that shares a cell or that joins a complexstr, and read an
empty cell as complexchar('\0'), as a narrow build already does.