Address PR feedback: reduce allocations and normalize empty edges

- Change RewriteId to return IEnumerable<string> using Enumerable.Repeat
  instead of allocating a new HashSet for every non-bare id
- Normalize empty edge sets to null after self-edge filtering to match
  the existing serialization convention in GraphTranslationUtility

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Aayush Maini

src/Microsoft.ComponentDetection.Orchestrator/Services/GraphTranslation/DefaultGraphTranslationService.cs

@@ -155,8 +155,9 @@ private static void ReconcileGraph(
         }
 
         // Rewrite a single Id: if it's a bare Id being merged, expand to its rich counterparts.
-        HashSet<string> RewriteId(string id) =>
-            bareToRich.TryGetValue(id, out var richIds) ? richIds : [id];
+        // Returns the existing set for bare ids; yields a single element for non-bare ids to avoid allocation.
+        IEnumerable<string> RewriteId(string id) =>
+            bareToRich.TryGetValue(id, out var richIds) ? richIds : Enumerable.Repeat(id, 1);
 
         // Rebuild graph: skip bare nodes being merged, rewrite edge targets.
         var newGraph = new Contracts.BcdeModels.DependencyGraph();
@@ -186,7 +187,7 @@ HashSet<string> RewriteId(string id) =>
                     }
                 }
 
-                newGraph[nodeId] = newEdges;
+                newGraph[nodeId] = newEdges.Count > 0 ? newEdges : null;
             }
         }
 
@@ -209,6 +210,12 @@ HashSet<string> RewriteId(string id) =>
                             }
                         }
                     }
+
+                    // Normalize empty edge sets to null for consistent serialization.
+                    if (newGraph[richId].Count == 0)
+                    {
+                        newGraph[richId] = null;
+                    }
                 }
             }
         }