fix(query): Do not execute filters if there are no source uids (#7962) (

#7969)

No need to execute filter subgraph if there are no source UIDs.

(cherry picked from commit 849b587)

query/query.go

@@ -2246,6 +2246,10 @@ func ProcessGraph(ctx context.Context, sg, parent *SubGraph, rch chan error) {
 			}
 
 			filter.SrcUIDs = codec.ToList(sg.DestMap)
+			if codec.ListCardinality(filter.SrcUIDs) == 0 {
+				filterChan <- nil
+				continue
+			}
 			// Passing the pointer is okay since the filter only reads.
 			filter.Params.ParentVars = sg.Params.ParentVars // Pass to the child.
 			go ProcessGraph(ctx, filter, sg, filterChan)