SelectInput reused the styled select and icon from NativeSelectInput,
so NativeSelect carried the open icon rotation and the iconOpen class
even though a native select never reports being open, and
MuiNativeSelect theme variants also styled the Select display and icon.
Give SelectInput its own styles, drop the open handling from
NativeSelectInput and document that NativeSelect never applies iconOpen.
Firefox applies the inherited letter-spacing to the zero-width space in
the unlabeled legend, which leaves a 0.15px gap in the border. Cap the
legend at `maxWidth: 0` when there is no label.
dualfroz