Should return Status::NotFound() when fullHistoryTsLow is empty

[CARROTINA]

fixes #11093

Added a check to see if the fullHistoryTsLow is empty as in ts_low->size() == 0 and returned Status::NotFound()

[ajkr]

Not sure. Empty full_history_ts_low seems to unambiguously mean none is set. There might need to be a reason to change it. Then if we do change it, we'll need a release note in HISTORY.md.

The original reason in #11093 is no longer valid as the assertion changed to permit empty string