do not report diagnosis "no peer detected" if no neighbor is detected but PDPortDataCheck.CheckPeers is not configured

After some intense discussions with specification and certification people the following definitionis agreed on:

1. If a portsubmodule shall perform a specific remote check (e.g. check MRPDomainUUID) and the check for a specific neighbor is configured via PDPortDataCheck.CheckPeers:
   1. if no neighbor is detected (Link down or no LLDP frame received), a diagnosis with "no peer detected" shall be issued and the specific diagnosis shall not be issued.
   2. if a peer is detected and the specific check fails (e.g. wrong MRPDomainUUID), a diagnosis for this specific check shall be issued
2. If a portsubmodule shall perform a specific remote check (e.g. check MRPDomainUUID) and the check for a specific neighbor is NOT configured via PDPortDataCheck.CheckPeers:
   1. if no neighbor is detected (Link down or no LLDP frame received), NO diagnosis shall be issued (neither "no peer detected" nor the specific diagnosis).
   2. if a peer is detected and the specific check fails (e.g. wrong MRPDomainUUID), a diagnosis for this specific check shall be issued

The current implementation does not respect 2.a and issues a diagnosis "no peer detected" in this case. This is wrong and shall be fixed.