Enable nullable reference types by default (#7130)

* Enable nullable reference types by default

This turns C# 9's nullable reference types on by default for all projects in the solution, and opts out unannotated files by adding `#nullable disable` to them.

This has two nice properties:

1. New code will be written with nullability in mind as the feature will be on by default.
2. It's easy to find unannotated code by searching for `#nullable disable`.

We have taken this approach in the .NET Project System and in CPS, and it's worked very well for us.

Code under `Deprecated` is unchanged.

Author: drewnoakes

src/Build.OM.UnitTests/AssemblyResources.cs

@@ -3,6 +3,8 @@
 
 using System;
 
+#nullable disable
+
 namespace Microsoft.Build.Shared
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ConstructionEditing_Tests.cs

@@ -15,6 +15,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ElementLocationPublic_Tests.cs

@@ -8,6 +8,8 @@
 using Xunit;
 using Shouldly;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectChooseElement_Tests.cs

@@ -13,6 +13,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectExtensionsElement_Tests.cs

@@ -9,6 +9,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectFormatting_Tests.cs

@@ -11,6 +11,8 @@
 using Xunit;
 using Xunit.Abstractions;
 
+#nullable disable
+
 namespace Microsoft.Build.Engine.OM.UnitTests.Construction
 {
     public class ProjectFormatting_Tests : IDisposable

src/Build.OM.UnitTests/Construction/ProjectImportElement_Tests.cs

@@ -13,6 +13,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectImportGroupElement_Tests.cs

@@ -10,6 +10,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectItemDefinitionElement_Tests.cs

@@ -8,6 +8,8 @@
 using Microsoft.Build.Exceptions;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>

src/Build.OM.UnitTests/Construction/ProjectItemDefinitionGroupElement_Tests.cs

@@ -9,6 +9,8 @@
 using InvalidProjectFileException = Microsoft.Build.Exceptions.InvalidProjectFileException;
 using Xunit;
 
+#nullable disable
+
 namespace Microsoft.Build.UnitTests.OM.Construction
 {
     /// <summary>