fix(ColorPicker): Fix default color value in ColorPicker (#6364)
* fix default color value to have full alpha channel
* add test
* make color required
* add missing color prop in RSP ColorPicker
* fix types
* fix tests
* add transparent example to colorswatch docs
* use previous value if input is empty/invalid
* add back null
* missed a few lines in tests
* fix types
* fix tests
* remove null from ColorPickerProps