Null check before walk to handle null values

[chajath]

I.e. previously edn.unify('{:x nil :y ?xxx}', '{:xxx 42}'); was failing due to null not having 'walk' as a member function. With this change I confirmed the said example works fine.

[chajath]

I won't commit jsedn.js as I am not sure what version of component we are using here. jsedn.js built from latest component generates file that are quite different in many places.