Heartbeats: Changes to the inactivity claim trigger

Here we introduce the following changes to the heartbeat inactivity claim trigger:

So far, inactivity claims have been issued with an empty inactive members set. This is a problem as the on-chain contract expects it to be non-zero. We comply with this requirement here.
Heartbeat signing errors are no longer counted as consecutive heartbeat inactivity failures. If signing fails, the most probable cause is the signing threshold that was not met. As an inactivity claim requires that threshold to be met as well, there is no sense in triggering that process in this case. Moreover, in case of a signing error, the inactive members set cannot be determined which violates the requirement from the first point

keep-network / keep-core