drivers: led: shell: avoid implicit conversion to boolean

Use a clear and style-adherent condition format.

Signed-off-by: Dmitrii Sharshakov <d3dx12.xx@gmail.com>

drivers/led/led_shell.c

@@ -29,7 +29,7 @@ static int parse_common_args(const struct shell *sh, char **argv,
 	char *end_ptr;
 
 	*dev = shell_device_get_binding(argv[arg_idx_dev]);
-	if (!*dev) {
+	if (*dev == NULL) {
 		shell_error(sh,
 			    "LED device %s not found", argv[arg_idx_dev]);
 		return -ENODEV;