Null value support

[vasilich6107]

Hi. Thanks for your work.

I'm experimenting with wrapping BlockPicker to use as reactive_forms control.
It seems that BlockPicker lacks of null value support for pickerColor value.

The form initial state could be empty(if creating a new record) - currently pickerColor: null causes an error so it requires to make pickerColor: value ?? Colors.transparent.

The other thing is lack of uncheck support for the picker.

[artoniaz]

My walk around is to pass to pickerColor any color value that isn't defined in your avaliableColors list.
However, my solution should thorw an error so I hope it's just a temporary solution.

Imo passing null value to pickerColor is a must, as starting with empy state is something obvoius.

Drowback of my solution.
In my case choosing a color is obligatory for a user. So primary I pass any random color that isn't on the list. On submiting the form I check if the valu has changed. You have to add such check also.