nat: Use observed addresses if we have no mappings from nat manager

[sukunrt]

If the user has selected libp2p.NATPortMap option, we attempt to do NAT port mapping using upnp and other protocols.
If we fail to detect the NAT or fail to do the port mapping our address inference logic via observed address field in identify completely fails.
What happens is that the check for natmgr here https://github.com/libp2p/go-libp2p/blob/master/p2p/host/basic/basic_host.go#L847 will be non nil and all calls to GetMapping will return nil.
To reproduce, run this in an ec2 instance with NatPortMap flag selected, or run this with a router where we are unable to do nat mapping.

This is a regression from v0.27. v0.27 did the right thing because it had this check https://github.com/libp2p/go-libp2p/blob/v0.27.0/p2p/host/basic/basic_host.go#L856

[sukunrt]

fixed by #2358

I'll open new issues for the edge cases that we can improve on. Those cases aren't regressions from v0.27, so I'm closing this.