Added getOrNull

Added get or null for uncertain data, which is pretty common in multiple sheets project

src/Model.php

@@ -54,6 +54,19 @@ public function get($item) {
         }
     }
 
+    /**
+    * Get value of property or null
+    * @param $item
+    * @return Mixed $item
+    */
+    public function getOrNull($item) {
+        if(!isset($this -> $item)) {
+            return null;
+        }else {
+            return $this -> $item;
+        }
+    }
+
     /**
      * Stores the $properties given to the instantiated object.
      * the $properties may came as an associative array or a stdClass object