zebra: accept LSP entries with an mpls-less outgoing interface

zebra: accept LSP entries with an mpls-less outgoing interface

The ZEBRA_MPLS_LABELS_[ADD/DELETE/REPLACE] messages may change an
LSP entry based on an incoming MPLS entry, followed by a given
next-hop.
Having a next hop with no label information inside is rejected
by the zebra layer. As illustration, the following ZAPI message
would be rejected, because the next hop does not contain any
label information.

  > ip -f mpls route add 105 via inet 192.0.2.45

At the same time, such configuration is desirable to be
supported:

An attempt has been done to configure the next-hop with an implicit-
null label. But the message is rejected by the kernel:

  > ip -f mpls route add 104 as 3 via inet 192.0.2.45
  > Error: Implicit NULL Label (3) can not be used in encapsulation.

The commit proposes to accept ZEBRA_MPLS_LABELS_[XX] messages with
a nexthop that does not contain any label information.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>