feat: Add builder with empty effect

[rubendm92]

Summary

I've added a function to create an empty effect ({}) in order to hide that the effects should be an object.
I think it is nice to hide that in case some day we wanted to change the effects to be an array, like in this PR #58

[mariosanchez]

Right now you can just return undefined, null or, in fact, any falsy value in an event handler

reffects/packages/reffects/src/index.js

Line 84 in 66d7ac6

if (!effects) {

does no this cover this purpose?

[rubendm92]

Ok, so if I have a super simple event handler I could do

registerEventHandler('foo', () => ({}))
registerEventHandler('foo', () => null)
registerEventHandler('foo', () => effects.none())

Returning null looks fine to me in this case, but if the event handler returns multiple effects and one of them is conditional like this

registerEventHandler('foo', (_, {successEvent}) => ({
    ...some.effect(),
    ...(successEvent
      ? effects.dispatch(successEvent.id, successEvent.payload)
      : {}),
  })
)

Opens a console, checks if spreading null works and, yep, it works

Aaaand here you could use null too 😅 I was expecting null to fail with the spread operator, but it works so ok. If we want to go for any falsy value is ok for me to close this PR 👍

[mariosanchez]

I wasn't covering this cases in my comment, but.. Ok xD If we think it's a good semantic addition, I'm okay with adding this anyway, since it's less dependent of knowing this particularity of JavaScript.