Fix a warning about an unnecessary test in gio/gdesktopappinfo.c

gio/gdesktopappinfo.c: In function ‘merge_directory_results’: gio/gdesktopappinfo.c:713:56: warning: the comparison will always evaluate as ‘true’ for the pointer operand in: ‘static_total_results + (sizetype)((long unsigned int)static_total_results_size * 16)’ must not be NULL [-Waddress] 713 | if (static_total_results + static_total_results_size != 0) | ^~

static_total_results is a pointer in memory and cannot be NULL, thus this test is not useful and can be safely removed.