Don't set highlight string if given value is undefined
Don't set highlight string if given value is undefined

When requesting more results for a given query,
we don't get the query string back from the dispatcher.
Therefore, the arrangement tries to set the highlight
string to be undefined. Guard against this!

https://phabricator.endlessm.com/T11270

endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib