fix eager loading of belongs_to relationships when all keys are null.

laravel/database/eloquent/relationships/belongs_to.php

@@ -71,6 +71,8 @@ public function eagerly_constrain($results)
 			}
 		}
 
+		if (count($keys) == 0) $keys = array(0);
+
 		$this->table->where_in($this->model->key(), array_unique($keys));
 	}