[5.4] Check if Collection is empty and prevent exception on fresh method. (laravel#19671)

* Add fresh method on Eloquent/Collection.

* Rewrite completely the method to avoid multiple database calls.
Add full tests for Model::fresh() and Collection::fresh() methods.

* Update tests for php5.6 and Carbon v1.20.0

* Check if collection is empty and prevent exception.

* styleCI

Author: mathieutu

tests/Database/DatabaseEloquentCollectionTest.php

@@ -357,6 +357,12 @@ public function testQueueableCollectionImplementationThrowsExceptionOnMultipleMo
         $c = new Collection([new TestEloquentCollectionModel, (object) ['id' => 'something']]);
         $c->getQueueableClass();
     }
+
+    public function testEmptyCollectionStayEmptyOnFresh()
+    {
+        $c = new Collection();
+        $this->assertEquals($c, $c->fresh());
+    }
 }
 
 class TestEloquentCollectionModel extends \Illuminate\Database\Eloquent\Model