Simplify condition, improve comment on this edge case.

Author: beberlei

src/UnitOfWork.php

@@ -3252,8 +3252,10 @@ private function eagerLoadCollections(array $collections, array $mapping): void
             foreach ($found as $targetValue) {
                 $sourceEntity = $targetProperty->getValue($targetValue);
 
-                // In cases where the hydration $targetValue has not yet fully completed
-                if ($sourceEntity === null && isset($targetClass->associationMappings[$mappedBy]['joinColumns'])) {
+                if ($sourceEntity === null) {
+                    // case where the hydration $targetValue itself has not yet fully completed, for example
+                    // in case a bi-directional association is being hydrated and deferring eager loading is
+                    // not possible due to subclassing.
                     $data = $this->getOriginalEntityData($targetValue);
                     $id   = [];
                     foreach ($targetClass->associationMappings[$mappedBy]['joinColumns'] as $joinColumn) {