API and checking if a custom field is empty is ignored

In the function &fixWhereValues() Line1538 of CRMContact/BAO/Query.php does a CRM_Utils_System::isNull() check on the parameters for the API request.

If an API parameter is something like != '' or > '' or >= '' that parameter is ignored meaning that the API can't be used to check if (custom?) fields are empty.

This is an issue because sets of custom fields become empty strings when only one/some of them are set.

Solution will probably require that empty strings are not ignored and are handled appropriately when creating the query.