fix problem with clearing non-empty location field after changing type of config variable

introduced by cd9db17 ("clear variable location cell in variable
panel, if variable class or type changed").

Closes #26

<?xml version='1.0' encoding='utf-8'?>
<BaseParams xmlns:xsd="http://www.w3.org/2001/XMLSchema" IEC_Channel="0" Name="py_ext_0"/>