model for blank input type=number field should be NaN

[chalin]

Currently a model property associated with an input of type=number is set to null when the field is plank. It should be set to NaN, consistent with the value returned by InputElement.valueAsNumber.

E.g., in Chrome yields:

Whereas yields:

You also get these same results when the input field contains any invalid text (i.e., text not recognized as a number).