Add validation on MailchimpSignupFormFormatter to ensure that entity is loaded before we access

Problem/Motivation

We have following for loop on class MailchimpSignupFormFormatter

    foreach ($items as $delta => $item) {
      if (!empty($item->_loaded)) {
        $entity = $item->entity;
      }

     // Other code
    }

Sometimes when $item->_loaded is empty, it tries to access the entity which is null.
We should continue if _loaded is empty.

Steps to reproduce

- Add a new signup form entity.
- Reference that on one block or paragraph field via entity reference field.
- Ensure that form is displayed when we view the content.
- Now delete the signup form from "admin/config/services/mailchimp/signup"
- Since deletion of signup form entity doesn't validate that signup is referenced anywhere on the page, it breaks the page where it is referenced.
- So in MailchimpSignupFormFormatter we should ensure that entity exists.

Proposed resolution

- Add a validation check on the formatter plugin.

Remaining tasks

- I've added an MR for the fix.
- Needs review

User interface changes

N/A

API changes

N/A

Data model changes

N/A