In GroupMembershipRequestForm make a check if the field_grequest_message is defined for the group content

Problem/Motivation

In issue https://www.drupal.org/project/social/issues/3453509 → we discussed removing the hook to automatically add the field_grequest_message. In order for that change to work we also need to remove any "hard-coded" assumptions that this field exist, since the hook will be remove we can no longer assume that the field will always be there. So we need to add a check if the field is present before populating it.

Steps to reproduce

1. Create a new group type and set the option to request to join for all users.
2. As a seperate user request the access to join the group and write a request message
3. As the group admin check if any request has been made and verify that the message is present.

Proposed resolution

Add a check if the field_grequest_message field exist in GroupRequestMembershipRequestForm.

Remaining tasks

User interface changes

API changes

Data model changes