bgpd: Check for L3VNI before sending RMAC/L3 RTs

bgpd: Check for L3VNI before sending RMAC/L3 RTs

Ensure that the presence of L3VNI is checked before we generate
Router MAC and L3 Route Target extended communities. Without this
check, the router would send an all-zeros RMAC in some situations,
which may cause problems for receivers.

Ticket: CM-21014
Testing Done:
a) Verification of failed scenario
b) Interop verification by Scott Laffer
c) evpn-smoke

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>