lldp: don't hard-code sysname, sysdescr and portdescr

lldp: don't hard-code sysname, sysdescr and portdescr

Instead of using "Not received" when we don't have them, just keep a
pointer to NULL. However, we need to handle that gracefully everywhere:
don't send them over SNMP, don't display them in lldpcli, be ready for
the fact that it should not be sent (even if this is not possible for
the local chassis), don't use it on places were we display the neighbor
and update tests.