Account for WhitespacePreservingTextInputWidget#getValue being called early

The new validation code in TextInputWidget causes getValue()
to be called from the InputWidget constructor, and this.whitespace
hasn't been initialized yet at this point.

Bug: 71246
Change-Id: Ib58812636f55eafef5c39468785c210cb0ac7827
Merge "Account for WhitespacePreservingTextInputWidget#getValue being called early"