Suppress nullable warnings in Serialization.Schema

Now that Schema compiles against the source assembly of System.CodeDom,
it receives nullability errors. I'm suppressing them manually for now
but am filing an issue to correctly fix those.

Related: #78036

src/libraries/System.Runtime.Serialization.Schema/src/System/Runtime/Serialization/Schema/CodeExporter.cs

@@ -518,8 +518,8 @@ private static CodeTypeDeclaration CreateTypeDeclaration(string typeName, DataCo
             CodeAttributeDeclaration generatedCodeAttribute = new CodeAttributeDeclaration(typeof(GeneratedCodeAttribute).FullName!);
 
             AssemblyName assemblyName = Assembly.GetExecutingAssembly().GetName();
-            generatedCodeAttribute.Arguments.Add(new CodeAttributeArgument(new CodePrimitiveExpression(assemblyName.Name)));
-            generatedCodeAttribute.Arguments.Add(new CodeAttributeArgument(new CodePrimitiveExpression(assemblyName.Version?.ToString())));
+            generatedCodeAttribute.Arguments.Add(new CodeAttributeArgument(new CodePrimitiveExpression(assemblyName.Name!)));
+            generatedCodeAttribute.Arguments.Add(new CodeAttributeArgument(new CodePrimitiveExpression(assemblyName.Version?.ToString()!)));
 
             // System.Diagnostics.DebuggerStepThroughAttribute not allowed on enums
             // ensure that the attribute is only generated on types that are not enums
@@ -818,7 +818,7 @@ private void ExportClassDataContract(DataContract classDataContract, ContractCod
             {
                 ContractCodeDomInfo baseContractCodeDomInfo = GetContractCodeDomInfo(classDataContract.BaseContract);
                 Debug.Assert(baseContractCodeDomInfo.IsProcessed, "Cannot generate code for type if code for base type has not been generated");
-                type.BaseTypes.Add(baseContractCodeDomInfo.TypeReference);
+                type.BaseTypes.Add(baseContractCodeDomInfo.TypeReference!);
                 AddBaseMemberNames(baseContractCodeDomInfo, contractCodeDomInfo);
                 if (baseContractCodeDomInfo.ReferencedTypeExists)
                 {
@@ -1151,7 +1151,7 @@ private void ExportISerializableDataContract(DataContract classDataContract, Con
             {
                 ContractCodeDomInfo baseContractCodeDomInfo = GetContractCodeDomInfo(classDataContract.BaseContract);
                 GenerateType(classDataContract.BaseContract, baseContractCodeDomInfo);
-                type.BaseTypes.Add(baseContractCodeDomInfo.TypeReference);
+                type.BaseTypes.Add(baseContractCodeDomInfo.TypeReference!);
                 if (baseContractCodeDomInfo.ReferencedTypeExists)
                 {
                     Type? actualType = (Type?)baseContractCodeDomInfo.TypeReference?.UserData[s_codeUserDataActualTypeKey];
@@ -1236,7 +1236,7 @@ private void ExportCollectionDataContract(DataContract collectionContract, Contr
             Debug.Assert(contractCodeDomInfo.TypeDeclaration != null);
 
             CodeTypeDeclaration generatedType = contractCodeDomInfo.TypeDeclaration;
-            generatedType.BaseTypes.Add(baseTypeReference);
+            generatedType.BaseTypes.Add(baseTypeReference!);
             CodeAttributeDeclaration collectionContractAttribute = new CodeAttributeDeclaration(GetClrTypeFullName(typeof(CollectionDataContractAttribute)));
             collectionContractAttribute.Arguments.Add(new CodeAttributeArgument(ImportGlobals.NameProperty, new CodePrimitiveExpression(dataContractName)));
             collectionContractAttribute.Arguments.Add(new CodeAttributeArgument(ImportGlobals.NamespaceProperty, new CodePrimitiveExpression(collectionContract.XmlName.Namespace)));
@@ -1671,7 +1671,7 @@ private static CodeThisReferenceExpression ThisReference
 
         private static CodePrimitiveExpression NullReference
         {
-            get { return new CodePrimitiveExpression(null); }
+            get { return new CodePrimitiveExpression(null!); }
         }
 
         private CodeParameterDeclarationExpression SerializationInfoParameter
@@ -1782,12 +1782,12 @@ private CodeMemberMethod GetSchemaStaticMethod
                         new CodeTypeReferenceExpression(GetCodeTypeReference(typeof(XmlSerializableServices))),
                         nameof(XmlSerializableServices.AddDefaultSchema),
                         new CodeArgumentReferenceExpression(paramDeclaration.Name),
-                        new CodeFieldReferenceExpression(null, TypeNameFieldName)
+                        new CodeFieldReferenceExpression(null!, TypeNameFieldName)
                     )
                 );
                 getSchemaStaticMethod.Statements.Add(
                     new CodeMethodReturnStatement(
-                        new CodeFieldReferenceExpression(null, TypeNameFieldName)
+                        new CodeFieldReferenceExpression(null!, TypeNameFieldName)
                     )
                 );
                 return getSchemaStaticMethod;