md_util: don't fetch unmerged MD range when the BID is null
md_util: don't fetch unmerged MD range when the BID is null

A queued CR can run after the local branch is cleared.  When it asks
for the unmerged MDs, folderBranchOps simply calls
`getUnmergedMDUpdates()` with whatever the current BID is -- which is
`NullBranchID`.  This ends up serving merged updates out of the MD
cache, which results in self-conflicts.  Instead, if the BID is null,
always serve back an empty list.

Issue: KBFS-2023

keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/kbfs,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client