[5.6] Let Arr::wrap(null) return empty array (#21745)

* Let Arr::wrap(null) return empty array

* Fix comment

* Update Arr.php

src/Illuminate/Support/Arr.php

@@ -594,13 +594,17 @@ public static function where($array, callable $callback)
     }
 
     /**
-     * If the given value is not an array, wrap it in one.
+     * If the given value is not an array and not null, wrap it in one.
      *
      * @param  mixed  $value
      * @return array
      */
     public static function wrap($value)
     {
+        if (is_null($value)) {
+            return [];
+        }
+
         return ! is_array($value) ? [$value] : $value;
     }
 }

tests/Support/SupportArrTest.php

@@ -662,5 +662,6 @@ public function testWrap()
         $this->assertEquals(['a'], Arr::wrap($string));
         $this->assertEquals($array, Arr::wrap($array));
         $this->assertEquals([$object], Arr::wrap($object));
+        $this->assertEquals([], Arr::wrap(null));
     }
 }