spectest: fix nil-deref generating the BeaconVoteDataNil valcheck vector

BeaconVoteDataNil builds a BeaconVote with nil Source/Target, then passed
*consensusData.Source / *consensusData.Target to NewSpecTest — which takes
phase0.Checkpoint by value — dereferencing the nil pointers and panicking
the whole vector generator (so no ssv spec-test vectors could be produced).

Pass zero checkpoints instead: a nil Source/Target SSZ-encodes to a zero
checkpoint, so that is the value the decoded vote carries, and the test
still asserts the source-not-less-than-target rejection.

Author: iurii-ssv

ssv/spectest/tests/valcheck/valcheckattestations/attestation_data_nil.go

@@ -1,14 +1,17 @@
 package valcheckattestations
 
 import (
+	"github.com/attestantio/go-eth2-client/spec/phase0"
+
 	"github.com/ssvlabs/ssv-spec/ssv/spectest/testdoc"
 	"github.com/ssvlabs/ssv-spec/ssv/spectest/tests"
 	"github.com/ssvlabs/ssv-spec/ssv/spectest/tests/valcheck"
 	"github.com/ssvlabs/ssv-spec/types"
 	"github.com/ssvlabs/ssv-spec/types/testingutils"
 )
 
-// BeaconVoteDataNil tests consensus data != nil
+// BeaconVoteDataNil tests value-check rejection of a BeaconVote with nil Source/Target (which encode to
+// zero checkpoints, so source is not less than target).
 func BeaconVoteDataNil() tests.SpecTest {
 	consensusData := &types.BeaconVote{
 		Source: nil,
@@ -23,8 +26,8 @@ func BeaconVoteDataNil() tests.SpecTest {
 		types.RoleCommittee,
 		testingutils.TestingDutySlot,
 		input,
-		*consensusData.Source,
-		*consensusData.Target,
+		phase0.Checkpoint{},
+		phase0.Checkpoint{},
 		nil,
 		nil,
 		types.AttestationSourceNotLessThanTargetErrorCode,