webrtc: add NullResourceManager, fixes panic

[dozyio]

Had issues dialling due to a null network.ResourceManager

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0xf76692]

goroutine 183 [running]:
github.com/libp2p/go-libp2p/p2p/transport/webrtc.(*WebRTCTransport).Dial(0xc00014d860, {0x16b4e00, 0xc000325c70}, {0x16c3dc0, 0xc000371e60}, {0xc00039ec60, 0x26})
	/root/go/pkg/mod/github.com/libp2p/[email protected]/p2p/transport/webrtc/transport.go:245 +0x52
github.com/libp2p/go-libp2p/p2p/net/swarm.(*Swarm).dialAddr(0xc000465208, {0x16b4e00, 0xc000325c70}, {0xc00039ec60, 0x26}, {0x16c3dc0, 0xc000371e60}, 0xc00026e900)
	/root/go/pkg/mod/github.com/libp2p/[email protected]/p2p/net/swarm/swarm_dial.go:545 +0x41d
github.com/libp2p/go-libp2p/p2p/net/swarm.(*dialLimiter).executeDial(0xc0004a6b90, 0xc0003e4dc0)
	/root/go/pkg/mod/github.com/libp2p/[email protected]/p2p/net/swarm/limiter.go:213 +0xfd
created by github.com/libp2p/go-libp2p/p2p/net/swarm.(*dialLimiter).addCheckFdLimit in goroutine 179
	/root/go/pkg/mod/github.com/libp2p/[email protected]/p2p/net/swarm/limiter.go:163 +0x485
exit status 2

[sukunrt]

Thanks @dozyio