gapplicationcommandline: Handle the possibility empty args in an example

While it’s not usual, it is possible for applications to be called with
an empty `argv` by a process (ab)using `execve()`. Modify the
`GApplication` code example to handle that possibility, so that anyone
copying the example isn’t introducing a bug into their program
immediately.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

gio/gapplicationcommandline.c

@@ -119,6 +119,12 @@
  *
  *   argv = *arguments;
  *
+ *   if (argv[0] == NULL)
+ *     {
+ *       *exit_status = 0;
+ *       return FALSE;
+ *     }
+ *
  *   i = 1;
  *   while (argv[i])
  *     {