Unpopulated events can raise errors during filtering

[diehlbw]

Problem Summary

[Creating from email reported issue] If an event type is unpopulated but defined, then it's possible to "successfully" merge in but later cause issue when the target (or other event type) is populated.

Impact

For locally developed + run notebooks this is likely an ok error as config can be pruned; however, if shared and being used across different populations then the user shouldn't need to do this.

Suggested fix

Log warning for empty events so the "deficiency" isn't hidden (in case prose referenece an option that has disappeared)
Trim target lists after merging, so they aren't selectable.

Alternatively, empty columns could be created - relying on each exploration to handle the 0rows case (likely falling into "censored" handling.