The OutlinedBorder to draw around the chip.
Defaults to the shape in the ambient ChipThemeData. If the theme shape resolves to null, the default is StadiumBorder.
This shape is combined with side to create a shape decorated with an outline. If it is a MaterialStateOutlinedBorder, MaterialStateProperty.resolve is used for the following MaterialStates:
@override final OutlinedBorder? shape;