Currently, when calculating the view update backlog for gossip,
we start with `db::view::update_backlog()` and compare it to backlogs
from all shards. However, this backlog can't be compared to other
backlogs - it has size 0 and we compare the fraction current/size
when comparing backlogs, causing us to compare with `NaN`.
This patch fixes it by starting the comparisons with an empty backlog.
The patch introducing this issue (f70f774e40) wasn't backported, so this one doesn't need to be either
Closesscylladb/scylladb#19247
* github.com:scylladb/scylladb:
mv: make the view update backlog unmofidiable
mv: fix value of the gossiped view update backlog
75441ee120