bug #23980 Tests and fix for issue in array model data in EntityType field with multiple=true (stoccc)

This PR was squashed before being merged into the 2.7 branch (closes #23980).

Discussion
----------

Tests and fix for issue in array model data in EntityType field with multiple=true

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| License       | MIT
| Fixed tickets | #23927

Provided some tests and the fix for #23927.
Rebased to 2.7, replaces #23930

Commits
-------

aaba6b4c2b Tests and fix for issue in array model data in EntityType field with multiple=true

Author: fabpot

Extension/Core/EventListener/MergeCollectionListener.php

@@ -80,7 +80,7 @@ public function onSubmit(FormEvent $event)
             return;
         }
 
-        if (!$dataToMergeInto) {
+        if (null === $dataToMergeInto) {
             // No original data was set. Set it if allowed
             if ($this->allowAdd) {
                 $dataToMergeInto = $data;