ephy-shell: Fix compilation warning

1481 ../src/ephy-shell.c: In function ‘webkit_notification_closed_cb’:
1482 /usr/include/glib-2.0/glib/glib-autocleanups.h:28:3: error: ‘id’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
1483    28 |   g_free (*pp);
1484       |   ^~~~~~~~~~~~
1485 ../src/ephy-shell.c:913:20: note: ‘id’ was declared here
1486   913 |   g_autofree char *id;