Is conditionally SQL Server log backup possible?
My idea is to count(*) the result of this function and if it increased then I'll take a log backup. But I'm not sure whether read will cause log record increase.


A read action, including reading from the system management views/functions should not cause any log activity, so this would work as you intend.

But why do you need to skip the log backups in this case? They will be minimally small as there has been no new activity since the last log backup so won't consume significant resources to take, store, transfer, and (if needed later) restore, and you would be adding an extra complication (so a potential point of failure) into your process.