DGML fails when there is unresolved dependency

src/lib/Microsoft.Fx.Portability.Reports.DGML/DGMLOutputWriter.cs

@@ -58,7 +58,7 @@ public Task WriteStreamAsync(Stream stream, AnalyzeResponse response)
                     dgml.AddNode(nodeGuid, nodeTitle,
                         nodeCategory,
                         portabilityIndex,
-                        group: missingTypes.Length == 0 ? null : "Collapsed");
+                        group: string.IsNullOrEmpty(missingTypes) ? null : "Collapsed");
 
                     if (dgml.TryGetId(tfm, out Guid frameworkGuid))
                     {

src/lib/Microsoft.Fx.Portability.Reports.DGML/ReferenceNode.cs

@@ -105,6 +105,12 @@ public bool TryGetAPICountFromReferences(int target, out int availAPIs, out int
 
             foreach (var item in Nodes)
             {
+                if (item.UsageData == null)
+                {
+                    // skip the Node with no UsageData
+                    continue;
+                }
+
                 if (!item.TryGetAPICountFromReferences(target, out int refCountAvail, out int refCountUnavail))
                 {
                     // Cycle!