Missing entries in covariate_balance_* files

[anthonysena]

From our offline discussion:

When reviewing side-by-side patient characterization tables in the Shiny app, we see seemingly random missing entries. This may be an issue in the Shiny app and possibly in Feature Extraction.

Per @jamieweaver: The problem is in the skeleton package in the export function where it compiles the balance files for the results data model.