[4.x] Fixed email is not valid spam

[indykoning]

Due to the v-model not being lazy debouncePromise is triggered on every character typed.
Which is not a problem for isEmailAvailable with the debounce. However handleGuest attempts to set the email on the cart, where validation is done wether the e-mail address is valid.

This means for every time an email is checked for availability it is also getting blocked by the setGuestEmailOnCart graphql call.

We do not want the v-model being lazy since the CheckoutLogin won't know the entered email until the field gets unfocussed.

see: #1258

[royduin]

Tests are failing? And master is v5 now, so we also need this on 4.x?

[indykoning]

Yes, we do want this on V4. I'll update it

The base branch was changed.

[royduin]

Please double check this one, also for v5 on master

[indykoning]

Ref: RAP-1815