B: Check if import field value is really NULL

When a value was 0, the clearfield symbol was also inserted, instead of 0.
Now this symbol is only inserted when the value is really empty.

app/code/community/Ho/Import/Model/Import.php

@@ -475,7 +475,7 @@ protected function _fieldMapItem(&$item)
                 }
 
                 if ($symbolForClearField && isset($fieldConfig['@'])
-                    && (!isset($itemRows[$storeCode][0][$fieldName]) || !$itemRows[$storeCode][0][$fieldName])) {
+                    && (!isset($itemRows[$storeCode][0][$fieldName]) || $itemRows[$storeCode][0][$fieldName] === NULL)) {
                     $itemRows[$storeCode][0][$fieldName] = $symbolForClearField;
                 }
             }