Added tests to verify that `null` and `undefined` are not acceptable keys
Added tests to verify that `null` and `undefined` are not acceptable keys

mweststrate/mobx-state-tree,mobxjs/mobx-state-tree,mobxjs/mobx-state-tree,mobxjs/mobx-state-tree,mobxjs/mobx-state-tree,mweststrate/mobx-state-tree