Fix NPE in FunctionalIndexSupport::loadFunctionalIndexDataFrame()

lookupCandidateFilesInMetadataTable(...) calls FunctionalIndexSupport::loadFunctionalIndexDataFrame() with an empty string for indexPartition which results in a NPE as loadFunctionalIndexDataFrame() tries to lookup and dereference index-definition using this empty string. 
 
This part of the code should never have worked - hence it looks like functional index (based on col-stats) is not tested on the query path. trying to get the index-partition to use on the query side seems more involved - the incoming query predicate needs to be parsed to get the (column-names, function-name) for all the query predicate and then fetch the corresponding index-partition by walking through the index-defs maintained in the index-metadata.