Undefined array key warnings in entity_form_display_alter() for an unconfigured rid

Problem/Motivation

### Problem/Motivation

The hook multiple_registration_entity_form_display_alter() dereferences $config[$rid] without checking that it exists, in both switch branches:

  $custom_form_mode = $config[$rid]['form_mode_register'] ?: 'register';
  $custom_form_mode = $rid ? $config[$rid]['form_mode_edit'] : 'default';

Each unconfigured $rid logs two warnings per request:

  Undefined array key "<rid>"
  Trying to access array offset on value of type null

This was fixed for the register branch in #3383084 (e88d0089), but the config read was later flattened from $config->get('roles')[$rid] to $config[$rid], and that rewrite reinstated ?: and lost the guard.

Steps to reproduce

1. Log in.
2. Request /user/register/.
3. Check /admin/reports/dblog: two warnings per request.

Proposed resolution

Restore the guards so the computed values are unchanged.

Remaining tasks

User interface changes

API changes

Data model changes