V10: fix more nullable references (#12321)

* Add Nullable and WarningsAsErrors to Build.props

* Remove Nullable from cs proj, add ImplicitUsings to build.props

* Fix errors in core

* Fix in JsonSchema

* Fix infrastructure

* Add non-ambiguous using

* Fix Web/Lucene

* Fix backoffice

* Fix up new build errors from merge

Co-authored-by: Nikolaj Geisle <niko737@edu.ucl.dk>

src/JsonSchema/NamespacePrefixedSchemaNameGenerator.cs

@@ -8,6 +8,6 @@ namespace JsonSchema
 {
     internal class NamespacePrefixedSchemaNameGenerator : DefaultSchemaNameGenerator
     {
-        public override string Generate(Type type) => type.Namespace.Replace(".", string.Empty) + base.Generate(type);
+        public override string Generate(Type type) => type.Namespace?.Replace(".", string.Empty) + base.Generate(type);
     }
 }