strv: Make sure strv_make_nulstr() always returns a valid nulstr

strv: Make sure strv_make_nulstr() always returns a valid nulstr

strv_make_nulstr() is documented to always return a valid nulstr,
but if the input is `NULL` we return a string terminated with only
a single NUL terminator, so let's fix that and always terminate the
resulting string with two NUL bytes.