fix: ensure push notification processor is never nil in newConn

- Add nil check in newConn to create VoidPushNotificationProcessor when needed
- Fix tests to use Protocol 2 for disabled push notification scenarios
- Prevent nil pointer dereference in transaction and connection contexts
- Ensure consistent behavior across all connection creation paths

The panic was occurring because newConn could create connections with nil
pushProcessor when options didn't have a processor set. Now we always
ensure a processor exists (real or void) to maintain the 'never nil' guarantee.

Author: ndyakov

push_notifications_test.go

@@ -174,9 +174,10 @@ func TestClientPushNotificationIntegration(t *testing.T) {
 }
 
 func TestClientWithoutPushNotifications(t *testing.T) {
-	// Test client without push notifications enabled
+	// Test client without push notifications enabled (using RESP2)
 	client := redis.NewClient(&redis.Options{
 		Addr:              "localhost:6379",
+		Protocol:          2,     // RESP2 doesn't support push notifications
 		PushNotifications: false, // Disabled
 	})
 	defer client.Close()
@@ -651,9 +652,10 @@ func TestPushNotificationProcessorConvenienceMethods(t *testing.T) {
 }
 
 func TestClientPushNotificationEdgeCases(t *testing.T) {
-	// Test client methods when processor is nil
+	// Test client methods when using void processor (RESP2)
 	client := redis.NewClient(&redis.Options{
 		Addr:              "localhost:6379",
+		Protocol:          2,     // RESP2 doesn't support push notifications
 		PushNotifications: false, // Disabled
 	})
 	defer client.Close()
@@ -673,10 +675,14 @@ func TestClientPushNotificationEdgeCases(t *testing.T) {
 		t.Errorf("Expected nil error when processor is nil, got: %v", err)
 	}
 
-	// GetPushNotificationProcessor should return nil
+	// GetPushNotificationProcessor should return VoidPushNotificationProcessor
 	processor := client.GetPushNotificationProcessor()
-	if processor != nil {
-		t.Error("Processor should be nil when push notifications are disabled")
+	if processor == nil {
+		t.Error("Processor should never be nil")
+	}
+	// VoidPushNotificationProcessor should have nil registry
+	if processor.GetRegistry() != nil {
+		t.Error("VoidPushNotificationProcessor should have nil registry when disabled")
 	}
 }
 

redis.go

@@ -970,8 +970,13 @@ func newConn(opt *Options, connPool pool.Pooler, parentHooks *hooksMixin) *Conn
 		c.hooksMixin = parentHooks.clone()
 	}
 
-	// Set push notification processor from options (always available now)
-	c.pushProcessor = opt.PushNotificationProcessor
+	// Set push notification processor from options, ensure it's never nil
+	if opt.PushNotificationProcessor != nil {
+		c.pushProcessor = opt.PushNotificationProcessor
+	} else {
+		// Create a void processor if none provided to ensure we never have nil
+		c.pushProcessor = NewVoidPushNotificationProcessor()
+	}
 
 	c.cmdable = c.Process
 	c.statefulCmdable = c.Process