Fix dependent root retrival genesis case

[terencechain]

#14039 fixed several issues related to duty retrieval. One key fix was to avoid heavily mutating the state for retrieving the beacon state proposer. However, after merging the PR, we realized it broke long-running end-to-end tests for the beacon API.

helpers.go:242: time="2024-05-24 19:14:42" level=error msg="error getting validator duties" error="failed to get duties for next epoch `1`: failed to get attester duties for epoch `1`: HTTP request unsuccessful (500: Could not get dependent root: could not get block root: slot 0 out of bounds)" prefix=client

Currently, getting the dependent root does not work in the genesis case. A genesis state does not have any dependent root when requesting at slot 0, as demonstrated by these two test cases:

func Test_attestationDependentRoot(t *testing.T) {
	st, _ := util.DeterministicGenesisState(t, 1)
	_, err := attestationDependentRoot(st, 0)
	require.NoError(t, err)
}

func Test_proposalDependentRoot(t *testing.T) {
	st, _ := util.DeterministicGenesisState(t, 1)
	_, err := proposalDependentRoot(st, 0)
	require.NoError(t, err)
}

This PR fixes the issue by using the genesis root in the beacon database if the genesis epoch is specified in the request.

[rkapka]

Left over from debugging?