Added nullability enable to all expected files

* Allows nullability annotations for any nullable types

Author: smaillet

src/Ubiquity.NET.CommandLine.SrcGen.UT/TestFiles/RootCommandAttributeTests/Basic_golden_path_succeeds/expected.cs

@@ -7,6 +7,8 @@
 //     the code is regenerated.
 // </auto-generated>
 // ------------------------------------------------------------------------------
+#nullable enable
+
 using static global::Ubiquity.NET.CommandLine.SymbolValidationExtensions;
 
 namespace TestNamespace

src/Ubiquity.NET.CommandLine.SrcGen.UT/TestFiles/RootCommandAttributeTests/DemoSource_succeeds/expected.cs

@@ -7,6 +7,8 @@
 //     the code is regenerated.
 // </auto-generated>
 // ------------------------------------------------------------------------------
+#nullable enable
+
 using static global::Ubiquity.NET.CommandLine.SymbolValidationExtensions;
 
 namespace TestNamespace

src/Ubiquity.NET.CommandLine.SrcGen.UT/TestFiles/RootCommandAttributeTests/Generator_handles_nullable_types/expected.cs

@@ -7,6 +7,8 @@
 //     the code is regenerated.
 // </auto-generated>
 // ------------------------------------------------------------------------------
+#nullable enable
+
 using static global::Ubiquity.NET.CommandLine.SymbolValidationExtensions;
 
 namespace TestNamespace