Unbound variable in SELECT clause should give empty column

[hannahbast]

Consider the following query, where the ?x in the SELECT clause is deliberately unbound:

PREFIX wdt: <http://www.wikidata.org/prop/direct/>
PREFIX wd: <http://www.wikidata.org/entity/>
SELECT ?x ?award_received WHERE {
  wd:Q873 wdt:P166 ?award_received
}

The correct result table would have an empty first column (because ?x is unbound) and second column containing all the awards (wdt:P166) of Mery Streep (wd:Q873). The Wikidata Query Service indeed outputs this.

QLever outputs the awards in the first column and leaves the second column empty. More generally, in a SELECT clause with x bound variable and y unbound variables, QLever (wrongly) produces a table with the x left columns filled with results.

I hope that this is not hard to fix. I need it for the autocompletion templates (if the prefix for the current predicate or entity is empty, we don't need to compute the aliases and then the alias variable is unbound and the respective column should simply be empty then).