support for sparse param for endpoints returning RCA

[alexandersimoes]

I am trying to use the sparse=true param for an endpoint returning RCAs and getting the following error:

Server error: `ERROR DB::Exception (47): DB::Exception: Missing columns: 'final_m0' while processing query: 

Here is an example URL: https://api.oec.world/tesseract/data.jsonrecords?cube=trade_i_baci_a_92&Exporter+Country=sapry&time=year.latest&drilldowns=Year&measures=Trade+Value&parents=true&rca=Exporter+Country,HS4,Trade+Value&sparse=true

And for the use case behind this: the networks on the OEC rely on the above data call but since they only return data when the given country exports that product we end up with a lot of "blanks" in the network viz.