[meshcop] ensure msg has timestamp TLV in `DatasetManager::HandleSet()` (#7023)
[meshcop] ensure msg has timestamp TLV in `DatasetManager::HandleSet()` (#7023)

This commit fixes `DatasetManager::HandleSet()` to add a check to
ensure that the message contains `PendingTimestampTlv` when the
Pending Dataset is being updated.
srickardti/openthread,abtink/openthread,openthread/openthread,jwhui/openthread,openthread/openthread,srickardti/openthread,srickardti/openthread,openthread/openthread,abtink/openthread,jwhui/openthread,openthread/openthread,jwhui/openthread,abtink/openthread,srickardti/openthread,jwhui/openthread,abtink/openthread