Map helpers: refactor setupDemeData
Map helpers: refactor setupDemeData

Refactor the function `setupDemeData` for better readability by
reducing the level of indentation of the code. This commit converts two
conditional checks into guard statements that exit the `forEach` loop
if each condition is not met.

Also removes an explicit `boolean === true` check in favor of `boolean`.

nextstrain/auspice,nextstrain/auspice,nextstrain/auspice