Rule: `no-implicit-undefined`

[igorkamyshev]

undefined used in Effector as a marker for skipping update in store. It can be hard to understand, if undefined is implicit used in store. E.g.

const $list = createStore([]);

const $item = $list.map(head);

This code will skip update if $list has an empty array.

The rule ought to forbid implicit usage of undefined in stores and mappers.