fix(serving): avoid returning a nil node in map (#4487)

kythe/go/serving/xrefs/xrefs.go

@@ -898,21 +898,23 @@ func d2d(d *srvpb.Document, patterns []*regexp.Regexp, nodes map[string]*cpb.Nod
 			continue
 		}
 
-		nodes[node.Ticket] = nodeToInfo(patterns, node)
+		n := nodeToInfo(patterns, node)
 		if def := node.DefinitionLocation; def != nil {
-			n := nodes[node.Ticket]
 			if n == nil {
 				// Add an empty NodeInfo to attach definition location even if no facts
 				// are requested.
 				n = &cpb.NodeInfo{}
-				nodes[node.Ticket] = n
 			}
 
 			n.Definition = def.Ticket
 			if _, ok := defs[def.Ticket]; !ok {
 				defs[def.Ticket] = a2a(def, false).Anchor
 			}
 		}
+
+		if n != nil {
+			nodes[node.Ticket] = n
+		}
 	}
 
 	return &xpb.DocumentationReply_Document{