The style to use for the hintText.
If hintStyle is a MaterialStateTextStyle, then the effective
text style can depend on the MaterialState.focused state, i.e.
if the TextField is focused or not.
Also used for the labelText when the labelText is displayed on
top of the input field (i.e., at the same location on the screen where
text may be entered in the InputDecorator.child).
If null, defaults to a value derived from the base TextStyle for the
input field and the current Theme.
final TextStyle? hintStyle;