To illustrate the issue before and after this fix:

    # /usr/bin/yad --list --print-all --column=A a '' c
    a|
    (null)|
    c|
    # ./src/yad --list --print-all --column=A a '' c
    a|
    |
    c|

The empty string '' in the second row results in a NULL C pointer,
which g_printf prints as "(null)". The fix consists of passing the
empty string "" to g_printf when the pointer is NULL.