Add "nullIsNothing"

[PaulJohnson]

Implements #35. "nullIsNothing" converts a Value prism into one that accepts "null" as "Nothing".

[paolino]

Hi, while you are at that I would suggest to add the other natural null handler

nullIsDefault :: q -> Prism' Value q -> Prism' Value q
nullIsDefault q p = prism'
  do review p 
  do \case 
      Null -> Just q
      x -> preview p x 

Thanks for picking this up

[PaulJohnson]

OK, done.

[ekmett]

This is covered by non Null using the base lens library.