suppress WHERE {} fixes #2

hello,

when I click Render Table mondrian sends empty slicer  axis <Axis name="SlicerAxis"> <Tuples> <Tuple/> </Tuples> </Axis> due to https://github.com/pentaho/mondrian/commit/88be25c4d9ed0f61035ae734193916be4abbdb72#L57R1995 , then xmla4j forms empty WHERE {} clause, when it sends back on drill down mondrian falls with 

Caused by: java.lang.NullPointerException
at mondrian.xmla.XmlaHandler$MDDataSet_Multidimensional.writeHierarchyInfo(XmlaHandler.java:2039)
at mondrian.xmla.XmlaHandler$MDDataSet_Multidimensional.axisInfo(XmlaHandler.java:2025)
at mondrian.xmla.XmlaHandler$MDDataSet_Multidimensional.olapInfo(XmlaHandler.java:1937)
at mondrian.xmla.XmlaHandler$MDDataSet_Multidimensional.unparse(XmlaHandler.java:1901)
at mondrian.xmla.XmlaHandler.execute(XmlaHandler.java:800)