CRM_Report_Form::selectTables() ignores filters for "null" and "not null"

The method CRM_Report_Form::selectTables() is supposed to build and return an array of tables used in the report, based on the Report Criteria settings. Currently, when checking report filters for tables that might be required, this method only counts the table if the filter has a defined value. This works fine for most filters, but the "null" and "not null" filters will never have a filter value; thus the tables they're associated with are not returned by this method.

This patch fixes the bug by counting a table as required if any of the three conditions is true: 1) the filter has a value, OR 2) the filter is "null", OR 3) the filter is "not null".