vxlan: MacLearning=false not being issued for networkd

When I have a config like:

  tunnels:
    vxlan42:
      mode: vxlan
      link: br42
      id: 42
      port: 4789
      local: 1.2.3.4
      mac-learning: false

the mac-learning variable is being ignored instead of making it into the .netdev file, which is bad, since the default is true, so there is no way to disable mac-learning currently. The responsible code is https://github.com/canonical/netplan/blob/main/src/networkd.c#L439-L440 , the fix would be to either drop the "if" and always set the parameter or to check for "!= NETPLAN_TRISTATE_UNSET" like in other locations.