Fix empty slot detection never firing - #89

Summary

getBlock returns a nil block with a nil error on a 404, but the only caller detecting empty slots checked the error message for "404" instead. Since the error was already nil by the time it got there, that branch was unreachable, so OnEmptySlot never fired and the beacon_empty_slots counter was permanently zero whenever DetectEmptySlots was enabled.

Fix checks the returned block instead of the error. Also pulled the check into its own method so it can be tested directly.

Test plan

• Added tests in pkg/beacon/empty_slot_test.go covering a missing block, an existing block, and detection disabled
• Confirmed the missing-block test fails against the old error-based check and passes against the fix
• go build ./..., go vet ./..., go test -race ./... all green