Fix error in column renderer when value is empty and not null (#4601)

* fix error in column renderer when valud is empty and not null

* test if operand is numeric

---------

Co-authored-by: Ng Kiat Siong <kiatsiong.ng@gmail.com>

Author: aamant

app/code/core/Mage/Adminhtml/Block/Widget/Grid/Column/Renderer/Number.php

@@ -32,7 +32,7 @@ class Mage_Adminhtml_Block_Widget_Grid_Column_Renderer_Number extends Mage_Admin
     protected function _getValue(Varien_Object $row)
     {
         $data = parent::_getValue($row);
-        if (!is_null($data)) {
+        if (is_numeric($data)) {
             $value = $data * 1;
             $sign = (bool) (int) $this->getColumn()->getShowNumberSign() && ($value > 0) ? '+' : '';
             if ($sign) {