Don't hydrate empty association entities.

When hasOne/belongsTo (joinable associations) are empty we should skip
populating empty entities as they cause problems later on in the
request.

Refs #3806
Refs #3902

src/ORM/ResultSet.php

@@ -400,14 +400,21 @@ protected function _groupResult($row) {
 
 			if ($assoc['canBeJoined']) {
 				$results[$alias] = $this->_castValues($target, $results[$alias]);
+
+				$hasData = array_filter($results[$alias], function ($v) {
+					return $v !== null;
+				});
+
+				if (!$hasData) {
+					continue;
+				}
 			}
 
 			if ($this->_hydrate && $assoc['canBeJoined']) {
 				$entity = new $assoc['entityClass']($results[$alias], $options);
 				$entity->clean();
 				$results[$alias] = $entity;
 			}
-
 			$results = $instance->transformRow($results, $alias, $assoc['canBeJoined']);
 		}