Fix for empty online source mapping

src/SourceBuild/content/eng/tools/tasks/Microsoft.DotNet.SourceBuild.Tasks.XPlat/UpdateNuGetConfigPackageSourcesMappings.cs

@@ -104,7 +104,7 @@ public override bool Execute()
                     foreach (var entry in oldSourceMappingPatterns)
                     {
                         // Skip sources with zero package patterns
-                        if (entry.Value != null)
+                        if (entry.Value?.Count > 0)
                         {
                             pkgSrcMappingClearElement.AddAfterSelf(GetPackageMappingsElementForSource(entry.Key, entry.Value));
                         }