feat: return an error for null rounds from RPC methods

[virajbhartiya]

Related Issues

Fixes #12633

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[virajbhartiya]

I have referenced the following for injecting null rounds in the itests,

lotus/itests/eth_transactions_test.go

Lines 432 to 454 in cfde1d5

	bms[0].InjectNulls(10)
	// wait until we produce blocks again
	tctx, cancel := context.WithTimeout(ctx, 30*time.Second)
	defer cancel()
	ch, err := client.ChainNotify(tctx)
	require.NoError(t, err)
	<-ch // current
	hc := <-ch // wait for next block
	require.Equal(t, store.HCApply, hc[0].Type)
	afterNullHeight := hc[0].Val.Height()
	nullHeight := afterNullHeight - 1
	for nullHeight > 0 {
	ts, err := client.ChainGetTipSetByHeight(ctx, nullHeight, types.EmptyTSK)
	require.NoError(t, err)
	if ts.Height() == nullHeight {
	nullHeight--
	} else {
	break
	}
	}

but getTipsetByBlockNumber is always resulting in requested a future epoch (beyond 'latest') and not returning the ErrNullRound error.

[virajbhartiya]

@rvagg the TestEthFeeHistory was failing because of now inclusion of null rounds, I have made most of the changes and the refactored calculateExpectations but it is still failing and I am unable to figure out why. Also addressed other comments

[rvagg]

👌 great work