drivers: stepper: shell: fix null pointer check

This commit introduces null pointer check in print_callback

Signed-off-by: Jilay Pandya <jilay.pandya@outlook.com>

drivers/stepper/stepper_shell.c

@@ -44,6 +44,9 @@ static void print_callback(const struct device *dev, const enum stepper_event ev
 			   void *user_data)
 {
 	const struct shell *sh = user_data;
+	if (!sh) {
+		return;
+	}
 
 	switch (event) {
 	case STEPPER_EVENT_STEPS_COMPLETED: