DX: fix some phpstan issues and add test for ParticipationOverlapValidator.php

src/Bundle/ChillThirdPartyBundle/Form/ChoiceLoader/ThirdPartyChoiceLoader.php

@@ -23,6 +23,8 @@ use function in_array;
 
 /**
  * Lazy load third parties.
+ *
+ * @deprecated As the PickThirdPartyType is deprecated, this should not be in use
  */
 class ThirdPartyChoiceLoader implements ChoiceLoaderInterface
 {
@@ -57,7 +59,7 @@ class ThirdPartyChoiceLoader implements ChoiceLoaderInterface
         $choices = [];
 
         foreach ($values as $value) {
-            if (empty($value)) {
+            if (null === $value || '' === $value) {
                 continue;
             }