fix(204): add #nullable enabled directive on top of generated file (#205)

fix(202): add `#nullable enabled` directive on top of generated file

This avoid CS8669 warnings

Signed-off-by: backtrack5r3 <flennertom@gmail.com>

Author: tomflenner

internal/cmd/testdata/success_csharp.golden

@@ -1,4 +1,5 @@
 // AUTOMATICALLY GENERATED BY OPENFEATURE CLI, DO NOT EDIT.
+#nullable enable
 using System;
 using System.Collections.Generic;
 using System.Threading.Tasks;

internal/generators/csharp/csharp.tmpl

@@ -1,4 +1,5 @@
 // AUTOMATICALLY GENERATED BY OPENFEATURE CLI, DO NOT EDIT.
+#nullable enable
 using System;
 using System.Collections.Generic;
 using System.Threading.Tasks;

test/csharp-integration/OpenFeature.cs

@@ -1,4 +1,5 @@
 // AUTOMATICALLY GENERATED BY OPENFEATURE CLI, DO NOT EDIT.
+#nullable enable
 using System;
 using System.Collections.Generic;
 using System.Threading.Tasks;

test/csharp-integration/OpenFeature.g.cs

@@ -1,4 +1,5 @@
 // AUTOMATICALLY GENERATED BY OPENFEATURE CLI, DO NOT EDIT.
+#nullable enable
 using System;
 using System.Collections.Generic;
 using System.Threading.Tasks;

test/csharp-integration/expected/OpenFeature.cs

@@ -1,4 +1,5 @@
 // AUTOMATICALLY GENERATED BY OPENFEATURE CLI, DO NOT EDIT.
+#nullable enable
 using System;
 using System.Collections.Generic;
 using System.Threading.Tasks;

test/csharp-integration/expected/OpenFeature.g.cs

@@ -1,4 +1,5 @@
 // AUTOMATICALLY GENERATED BY OPENFEATURE CLI, DO NOT EDIT.
+#nullable enable
 using System;
 using System.Collections.Generic;
 using System.Threading.Tasks;