R1:d254c1f8b14e

Use "null", not "-1", as a local "no version" marker when performing intracluster repository sync

Summary:
Ref T13242. See <https://discourse.phabricator-community.org/t/out-of-range-value-for-column-deviceversion/2218>.

The synchronization log column is `uint32?` and `-1` doesn't go into that column.

Since we're only using `-1` for convenience to cheat through `$max_version > $this_version` checks, use `null` instead and make the checks more explicit.

Test Plan: Reproducing this is a bit tricky and I…