Early fail on 0 active validators when computing sync committees (#7585)

Author: zilm13

ethereum/spec/src/main/java/tech/pegasys/teku/spec/logic/versions/altair/helpers/BeaconStateAccessorsAltair.java

@@ -101,6 +101,8 @@ public IntList getNextSyncCommitteeIndices(final BeaconState state) {
     final UInt64 epoch = getCurrentEpoch(state).plus(1);
     final IntList activeValidatorIndices = getActiveValidatorIndices(state, epoch);
     final int activeValidatorCount = activeValidatorIndices.size();
+    checkArgument(activeValidatorCount > 0, "Provided state has no active validators");
+
     final Bytes32 seed = getSeed(state, epoch, Domain.SYNC_COMMITTEE);
     int i = 0;
     final SszList<Validator> validators = state.getValidators();