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.
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