agent: fix SNMP walk on lldpRemTable when missing remote sysName

agent: fix SNMP walk on lldpRemTable when missing remote sysName

When enumerating lldpRemSysName (and some others), one row could have
a NULL value because the remote system didn't provide a value. In this
case, we should return the next row.

There was already some code around that but it was not systematically
used. Therefore, we fix the issue for lldpRemTable and
lldpLocalSystemData. To ensure we catch future cases, we ensure
helpers functions use `default: return NULL` when no missing value is
allowed (no `break`, compiler would catch if it was the case) and
therefore, we don't need to try next OID and `default: break` when a
value may be missing and in this case, the caller should try next OID
upon receiving NULL.

Fix #392