Allow null handling with @InitBinder & PropertyEditor/Converter/Formatter to handle null values.

With @InitBinder it is currently possible to register various property handlers to customize the web request data binding via binder.registerCustomEditor(Class<?>, String, PropertyEditor), binder.addCustomFormatter(Formatter<?>, String...) or using a Converter.
However none of these get called if the property is absent from the request.

I don't know if this is a bug or desired behavour, but there should be at least the possibility to handle such cases.

Current Behavior
Neither registered PropertyEditor nor Formatter nor Converter get invoked when a property is missing from the request. That is it only works if the property is explicitly sent as `null in the request.

Desired Behavior
Allow for PropertyEditor / Formatter / Converter to get called if an Optional/@Nullableproperty is absent from the request. And not just when explicitely sent asnull.

Start with Spring MVC request data binding and the @InitBinder registration points named in the issue: registerCustomEditor, addCustomFormatter, and Converter handling. Reproduce a request where an Optional or @Nullable property is absent, then compare it with an explicit null value. Done means the registered PropertyEditor, Formatter, or Converter can handle the absent-property case without changing existing binding behavior.