Error on duties calculation on low amount of validators setup

This issues happens when we have a small amount of validators in the setup (<10), this is important because this is the most common scenario.

Here are the logs:

shutdown: failed to start child: LambdaEthereumConsensus.Beacon.BeaconNode
      ** (EXIT) an exception was raised:
          ** (ArgumentError) errors were found at the given arguments:
  
    * 1st argument: the table identifier does not refer to an existing ETS table
  
              (stdlib 5.2.3) :ets.member(:beacon_committee, {0, {0, <<0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0>>}})
              (lambda_ethereum_consensus 0.1.0) lib/lambda_ethereum_consensus/state_transition/cache.ex:87: LambdaEthereumConsensus.StateTransition.Cache.present?/2
              (lambda_ethereum_consensus 0.1.0) lib/lambda_ethereum_consensus/state_transition/accessors.ex:381: LambdaEthereumConsensus.StateTransition.Accessors.maybe_prefetch_committees/2
              (lambda_ethereum_consensus 0.1.0) lib/lambda_ethereum_consensus/validator/duties.ex:93: anonymous fn/4 in LambdaEthereumConsensus.Validator.Duties.compute_duties_for_epochs/4
              (elixir 1.16.2) lib/enum.ex:2528: Enum."-reduce/3-lists^foldl/2-0-"/3
              (lambda_ethereum_consensus 0.1.0) lib/lambda_ethereum_consensus/validator/validator_set.ex:191: LambdaEthereumConsensus.ValidatorSet.compute_duties/4
              (lambda_ethereum_consensus 0.1.0) lib/lambda_ethereum_consensus/beacon/beacon_node.ex:30: LambdaEthereumConsensus.Beacon.BeaconNode.init/1
              (stdlib 5.2.3) supervisor.erl:330: :supervisor.init/1 
  ** (exit) :terminating
      (kernel 9.2.4) application_controller.erl:505: :application_controller.call/2
      (kernel 9.2.4) application.erl:155: :application."-ensure_all_started/3-lc$^0/1-0-"/1
      (kernel 9.2.4) application.erl:155: :application.ensure_all_started/3
      (mix 1.16.2) lib/mix/tasks/app.start.ex:72: Mix.Tasks.App.Start.start/3
      (mix 1.16.2) lib/mix/task.ex:478: anonymous fn/3 in Mix.Task.run_task/5
      (mix 1.16.2) lib/mix/tasks/run.ex:129: Mix.Tasks.Run.run/5
      (mix 1.16.2) lib/mix/tasks/run.ex:85: Mix.Tasks.Run.run/1
      (mix 1.16.2) lib/mix/task.ex:478: anonymous fn/3 in Mix.Task.run_task/5

Reproduce the failure with fewer than 10 validators, then start at lib/lambda_ethereum_consensus/state_transition/cache.ex:87 and follow the calls through accessors.ex:381 and validator/duties.ex:93. Check how the :beacon_committee ETS table is used during BeaconNode initialization and validator_set.ex:191. Done means the small-validator setup starts without the ETS-table error and the regression is covered.