fix: a possible crash in event target Close() (#17948)

these are possible crashes when the configured
target is still in init() state and never finished
- however a delete config was initiated.

internal/event/target/mysql.go

@@ -312,7 +312,11 @@ func (target *MySQLTarget) Close() error {
 		_ = target.insertStmt.Close()
 	}
 
-	return target.db.Close()
+	if target.db != nil {
+		return target.db.Close()
+	}
+
+	return nil
 }
 
 // Executes the table creation statements.