fixed potential NPE in XLSSchemaReader

Change-Id: Ie2369021587b356504f75c7a8baf36cd03c56aa4
Reviewed-on: http://gerrit.esdi-community.eu/3134
Tested-by: Jenkins <sitemple@igd.fraunhofer.de>
Reviewed-by: Simon Templer <simon.templer@igd.fraunhofer.de>