bug: metadata might not always be available

[romanzac]

Describe the bug
During TestHandlePeerTopicEvent() execution nil pointer error occurred. Problem is with newly added Waku metadata check in handlerPeerTopicEvent(). This check pm.metadata.RelayShard() assumes metadata are always set.

To Reproduce

1. Checkout f8a67f8 from chore: peermanager tests coverage improvement #1035
2. Run go test

Expected behavior
Please add nil metadata situation handling or ensure metadata are never nil with example how to instantiate when no cluster/sharding has been created.

Test run log

2024-03-15T10:08:35.464+0800	INFO	gowaku.relay	relay/waku_relay.go:247	gossipsub subscription	{"pubsubTopic": "/waku/2/go/pm/test"}
2024-03-15T10:08:35.466+0800	INFO	gowaku.peer-manager	peermanager/topic_event_handler.go:25	handleNewRelayTopicSubscription	{"pubSubTopic": "/waku/2/go/pm/test"}
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x40 pc=0x101377e58]

goroutine 2424 [running]:
github.com/waku-org/go-waku/waku/v2/protocol/metadata.(*WakuMetadata).RelayShard(0x5?)
	/Users/roman/sources/waku-org/go-waku/waku/v2/protocol/metadata/waku_metadata.go:82 +0x18
github.com/waku-org/go-waku/waku/v2/peermanager.(*PeerManager).handlerPeerTopicEvent(0x1400038fad0, {{0x1017c17b6, 0x12}, {0x14001d67560, 0x22}, 0x0})
	/Users/roman/sources/waku-org/go-waku/waku/v2/peermanager/topic_event_handler.go:124 +0x42c
github.com/waku-org/go-waku/waku/v2/peermanager.(*PeerManager).peerEventLoop(0x1400038fad0, {0x101d54bf8, 0x1400011e018})
	/Users/roman/sources/waku-org/go-waku/waku/v2/peermanager/topic_event_handler.go:174 +0x1e0
created by github.com/waku-org/go-waku/waku/v2/peermanager.TestHandlePeerTopicEvent
	/Users/roman/sources/waku-org/go-waku/waku/v2/peermanager/topic_event_handler_test.go:182 +0x5c8
exit status 2
FAIL	github.com/waku-org/go-waku/waku/v2/peermanager	8.491s