Nullable annotations for System.Runtime.Serialization.Xml and System.Runtime.Serialization.Json

[eerhardt]

Contributes to #2339

cc @jeffhandley @terrajobst

[Dotnet-GitSync-Bot]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[eerhardt]

fyi - @HongGit @StephenMolloy - as the owners of this area.

[jozkee]

Unrelated to this PR: There is a file that doesn't even compile in System.Private.DataContractSerialization and is a duplicate of the homonymous named file CollectionDataContractAttribute.cs in System.Runtime.Serialization.Primitives, I think that file can be removed (on a separate PR):

src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/CollectionDataContractAttribute.cs

src/libraries/System.Runtime.Serialization.Primitives/src/System/Runtime/Serialization/CollectionDataContractAttribute.cs

[krwq]

[DisallowNull]?

[eerhardt]

How does [DisallowNull] work with overrides? This one is a bit tricky because the base class has:

class DataContract
{
    XmlDictionaryString _name; // can't be null

    public XmlDictionaryString Name { get { } set{ _name = value; } }

    public virtual XmlDictionaryString? TopLevelElementName
    {
        get { return _name; } set{ _name = value; }
    }
}

but then a derived class does:

class DerivedDataContract : DataContract
{
    XmlDictionaryString? _topLevelName ; // can be null

    public override XmlDictionaryString? TopLevelElementName
    {
        get { return _topLevelName ; } set{ _topLevelName = value; }
    }
}

And when trying to clone the objects, now we have a problem:

runtime/src/libraries/System.Private.DataContractSerialization/src/System/Runtime/Serialization/ClassDataContract.cs

Lines 1508 to 1509 in 6072e4d

	clonedHelper.TopLevelElementName = this.TopLevelElementName;
	clonedHelper.TopLevelElementNamespace = this.TopLevelElementNamespace;

The getter can be null, but the setter doesn't allow for it.

[krwq]

public override void WriteStartAttribute(string prefix, string localName) and overload might be worth filing a bug on NRE or at least TODO-NULLABLE

[eerhardt]

The WriteStartAttribute methods are internal and are never called with null for prefix, so I left them as non-nullable and didn't change the code. But WriteXmlnsAttribute could be called with null, so that's why I fixed it here.

[krwq]

LGTM after fixing/resolving comments

[jozkee]

Otherwise; public API annotations LGTM.

[eerhardt]

I believe all current feedback has been addressed. I left a few conversations open with pending questions. Please let me know if you have any more feedback. I'd like to merge this by the EOD.