Last Day Worked variable behaviour

In the old process, if the "Last Day Worked" variable was blank, within the app I think it defaults to a filler of todays date regardless of what state an employee is in, i.e., everyone that had a blank has 2023-02-28 in their record when looking in the app. When exporting the data, these actually appear as "Invalid date". In the new process, it looks like it is defaulting blank dates to 1900-01-01, which both appears in the app and in the exported data.

This might merit further conversation, possibly with the PSA as well, but maybe we want to consider handling blanks differently (perhaps, it's a calculation of one day before effective date if blank?)

No files, tests, or entry points are named. First clarify the intended behavior for blank "Last Day Worked" values with the project and PSA, then trace the old and new date handling in both the app and export paths; done means an agreed blank-date result appears consistently in both places.

Written by the indexing model from the issue text.