EDA Backend: new endpoint data service for "at least one non-null value" for every variable in current subset

[danicahelb]

Backend engineering of an EDA data service that will evaluate a "has at least one non-null value" boolean for every variable in the study for the current subset

We will add button to all viz trees to "audit" which variables have at least one value (see frontend ticket: #725).

use cases: (1) sparse VB megastudy, (2) Artemesinin resistance surveillance consortium (new grant we will submit in Jan 24)