the LastHeartbeatTS may lost sometime

Found that when a tikv disconnect or is not available for a long time, the LastHeartbeatTS which the pd's api: pd/api/v1/stores return is nil.
More plausibly, the last hearbeat time of tikv should always be preserved, no matter how long it disconnect and what status it become.

Start by tracing the pd/api/v1/stores endpoint and the code that populates LastHeartbeatTS when a TiKV store disconnects or becomes unavailable. Reproduce or inspect the unavailable-store response, then verify that the previously recorded heartbeat timestamp remains present regardless of the store status.

Written by the indexing model from the issue text.