Discovery: don't suppress response when merging for non-merge triggered discovery requests

In Discovery, when receiving a discovery request, we discard it if a merge is running (isMergeRunning()). The condition should be moved into the if-clause above (hdr.view_id != null), as it only applies to merge-triggered discoveries. When handling a regular discovery, e.g. at startup or when a physical address is looked up, a discovery response should be sent.