Calculate correctly the height of a TextFieldView's hint when the content is set to null

If a text field is built with a non empty value, the size of the hint is not updated correctly when the field's value is set to null or when the field is reused. This patch simply calls _fixupTextLayout() on render->update branch.

I added also a testcase but couldn't check it. For unknown reasons I no longer can run testcases on the dev branch. Anything changed here?