Panic on validator syncher when processing old blocks

[ylembachar]

Description The keyper panics during validator registry sync when a registration event results in an empty pubKeys slice. This seems to occur when syncing from older block heights and seems related to recent changes to validator registration handling.

Logs

2025/05/19 09:54:51.689029 WRN [validatorsyncer.go:186] ignoring registration message with invalid nonce block-hash=213a3f2770e58a52f6d0a59b289211599ed40f7332b07d1449d9efd75ebea850 block-number=34879139 latest-nonce=0 log-index=9 nonce=0 tx-index=2 validator-index=238930
panic: runtime error: index out of range [0] with length 0

goroutine 54 [running]:
github.com/shutter-network/rolling-shutter/rolling-shutter/keyperimpl/gnosis.(*ValidatorSyncer).filterEvents(0xc0004e15f0, ...)
	/src/rolling-shutter/keyperimpl/gnosis/validatorsyncer.go:220 +0x1026
...
2025-05-19 09:54:51,694 WARN exited: keyper (exit status 2; not expected)