Bluetooth: shell: Fix requiring UUID for gatt-discover

This enables using these commands to perform discover all procedure:

> gatt-discover-primary
 Discover pending
 Service 1800 found: start handle 1, end_handle 5
 Service 1801 found: start handle 6, end_handle 9

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>