Remove singleton empty collections

They were only used in test code, and they don't add any value over `[]`.

Author: drewnoakes

src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/PackageRestore/Snapshots/RestoreBuilder.cs

@@ -9,9 +9,6 @@ namespace Microsoft.VisualStudio.ProjectSystem.PackageRestore;
 /// </summary>
 internal static class RestoreBuilder
 {
-    public static readonly ImmutableArray<TargetFrameworkInfo> EmptyTargetFrameworks = [];
-    public static readonly ImmutableArray<ReferenceItem> EmptyReferences = [];
-
     /// <summary>
     ///     Converts an immutable dictionary of rule snapshot data into an <see cref="ProjectRestoreInfo"/> instance.
     /// </summary>

tests/Microsoft.VisualStudio.ProjectSystem.Managed.UnitTests/Mocks/ProjectRestoreInfoFactory.cs

@@ -6,8 +6,11 @@ internal static class ProjectRestoreInfoFactory
 {
     public static ProjectRestoreInfo Create(string? msbuildProjectExtensionsPath = null)
     {
-        return new ProjectRestoreInfo(msbuildProjectExtensionsPath ?? string.Empty, string.Empty, string.Empty,
-                                      RestoreBuilder.EmptyTargetFrameworks,
-                                      RestoreBuilder.EmptyReferences);
+        return new ProjectRestoreInfo(
+            msbuildProjectExtensionsPath: msbuildProjectExtensionsPath ?? "",
+            projectAssetsFilePath: "",
+            originalTargetFrameworks: "",
+            targetFrameworks: [],
+            toolReferences: []);
     }
 }