Add debug type tests and allow undefined shape in reshape

[sergeysova]

1. Allow to return undefined from shape in reshape method:

const source = createStore<{ last?: string, first: string }>({ first: "hi" })

const { first, last } = reshape({
  source,
  shape: {
    first: (data) => data.first,
    last: (data) => data.last,
  },
})

last.getState() // null

2. Cover debug with type tests