Fix "unable to select ANY" in product association grid (#2182)

Author: fballiano

app/code/core/Mage/Adminhtml/Block/Widget/Grid.php

@@ -1298,18 +1298,12 @@ public function getParam($paramName, $default = null)
     {
         $session = Mage::getSingleton('adminhtml/session');
         $sessionParamName = $this->getId().$paramName;
-        $param = $this->getRequest()->getParam($paramName);
-
-        if ($param !== null) {
-            if (trim($param) !== '') {
-                if ($this->_saveParametersInSession) {
-                    $session->setData($sessionParamName, $param);
-                }
-                return $param;
-            } else {
-                $session->unsetData($sessionParamName);
-                return $default;
+        if ($this->getRequest()->has($paramName)) {
+            $param = $this->getRequest()->getParam($paramName);
+            if ($this->_saveParametersInSession) {
+                $session->setData($sessionParamName, $param);
             }
+            return $param;
         }
         elseif ($this->_saveParametersInSession && ($param = $session->getData($sessionParamName)))
         {