custom String converters disallowed in myfaces implementation?

[Mike Kienenberger]

I notice that Myfaces disallows custom String (and Object) converters.

===
static Converter findUIOutputConverter(FacesContext facesContext,
UIOutput component)
        if (String.class.equals(valueType)) return null;    //No
converter needed for String type
        if (Object.class.equals(valueType)) return null;    //There is
no converter for Object class
===

I don't see anything in the spec that requires this restriction.

My use case is that I would like to create a java.lang.String
converter that automatically converts all empty strings to null.