fix(sentry): Panic fix in wallclock

[samcm]

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0xfa8daa]

goroutine 55 [running]:
github.com/ethpandaops/ethwallclock.(*EthereumBeaconChain).OnEpochChanged(...)
	/home/runner/go/pkg/mod/github.com/ethpandaops/ethwallclock@v0.3.0/beacon_chain.go:92
github.com/ethpandaops/xatu/pkg/sentry/ethereum/services.(*DutiesService).Start(0xc000431960, {0x1775fe8?, 0xc000050018})
	/home/runner/work/xatu/xatu/pkg/sentry/ethereum/services/duties.go:78 +0x12a
github.com/ethpandaops/xatu/pkg/sentry/ethereum.(*BeaconNode).Start.func1()
	/home/runner/work/xatu/xatu/pkg/sentry/ethereum/beacon.go:77 +0x323
created by github.com/ethpandaops/xatu/pkg/sentry/ethereum.(*BeaconNode).Start
	/home/runner/work/xatu/xatu/pkg/sentry/ethereum/beacon.go:61 +0xf8