fix: [2.5] Return early when skip load pk index (#39763)

Cherry pick from master
pr: #39762
Previous PR #39437 only print log and add index while load operation is
still executed. This PR return early when segment decides not to load PK
index.

Signed-off-by: Congqi Xia <congqi.xia@zilliz.com>

internal/querynodev2/segments/segment.go

@@ -1033,6 +1033,7 @@ func (s *LocalSegment) LoadIndex(ctx context.Context, indexInfo *querypb.FieldIn
 			IndexInfo: indexInfo,
 			IsLoaded:  true,
 		})
+		return nil
 	}
 
 	return s.innerLoadIndex(ctx, fieldSchema, indexInfo, tr, fieldType)