dispatcher: Fix panic due to nil tableInfo in ddl event (#3670)

close #3669

Author: hongyunyan

pkg/common/event/util.go

@@ -494,6 +494,11 @@ func BatchDML(dml *DMLEvent) *BatchDMLEvent {
 // Only the table with pk and no uk can always be splitted in all sinks.
 // Notice: please ensure the logic of IsSplitable is totally the same with isSplitable in utils
 func IsSplitable(tableInfo *common.TableInfo) bool {
+	// some ddl jobs do not have table info, such as drop database, we just ignore checking these jobs
+	if tableInfo == nil {
+		return true
+	}
+
 	if tableInfo.GetPkColInfo() == nil {
 		return false
 	}