Upgrade script (3.1.3) should check for NULL values for activity type name column and fix if present

Some existing installs have NULL values in the civicrm_option_value.name column for 'custom' activity types. This causes a fatal error on the activity selector and dashboard widget. We should check for this condition in the 3.1.3 upgrade script and fix if found (by copying the label value to the name column).

NOTE: This fix should be applied only to option_value rows belonging to the activity_type option group.