ts: issuie with merging break-point ts with interior fragment

Overview

As pointed out by @jehelset, merge(ts_a,ts_b) where ts_b is a break point ts, had issue at the end of ts_a.total_period().end, not handled correctly, if ts_b lacked point at the same time-point.

This could easily happen during compute, or cache-update(resulting in a different cache than stored break point ts).