Investigate the performance of empty childDataFrame in FLWOR clauses

[CanBerker]

@ghislainfourny Should the short circuit mentioned above (seen in OrderByClauseSparkIterator.java file) be added to other FLWOR clauses as well? It could save a bit of unnecessary computation and querying when operating on empty sequences. However this introduces an additional count() action which might be costly.

Here I think performance measurements would tell which is better. For an empty DF, the closures would not be called anyway, so that the count() action may be more costly than the gain. Maybe you could open an issue on this?

Originally posted by @ghislainfourny in #394 (comment)