executor: fix unstable unit test TestMergeJoinInDisk (#54524)

close #54006

pkg/executor/sortexec/sort.go

@@ -340,7 +340,7 @@ func (e *SortExec) generateResultWithMultiWayMerge() error {
 func (e *SortExec) generateResultFromDisk() error {
 	inDiskNum := len(e.Parallel.spillHelper.sortedRowsInDisk)
 	if inDiskNum == 0 {
-		panic("inDiskNum can't be 0 when we generate result with spill triggered")
+		return nil
 	}
 
 	// Spill is triggered only once