Validate the redirect URL only when "Enable URL redirect" is selected

I will also verify what Drupal core does in similar cases.

I also simplified the code, since $form_state->hasValue() is usually not called before calling $form_state->getValue().

Hello all,
I have applied the MR from here and followed the mentioned steps of this issue, now it's look like the problem is fixed now, moving is to RTBC. I am attaching the screencast also.

The same change should be done for the Drupal 7 branch.

I committed the changes directly to the project branch because the issue fork branch was not updated and I was not able to update it. (The issue fork UI allows to update the fork only from the default branch, which is not the Drupal 7 branch.)

apaderno → changed the visibility of the branch 3454259-d7-validate-the-redirect-url-only-when-necessary to hidden.

Automatically closed - issue fixed for 2 weeks with no activity.