dsApiRequest: make skip argument optional (error with facets)

[pgitti]

When querying for facets, offset parameters, i.e. "skip" are not supported. However, dsApiRequest automatically adds skip.

E.g.
search publications where research_orgs in ["grid.10420.37"] and year>2021 return year limit 1000 skip 100
-> Semantic errors found: Offset is not supported for facet results

search publications where research_orgs in ["grid.10420.37"] and year>2021 return year limit 1000
-> works