Add nil check to test

I've seen the following flaky error in CI:
```
panic: runtime error: invalid memory address or nil pointer dereference
/opt/hostedtoolcache/go/1.17.5/x64/src/testing/testing.go:1209 +0x24e
/opt/hostedtoolcache/go/1.17.5/x64/src/testing/testing.go:1212 +0x218
/opt/hostedtoolcache/go/1.17.5/x64/src/runtime/panic.go:1038 +0x215
/runner/_work/celo-blockchain/celo-blockchain/core/types/block.go:487 +0x30
/runner/_work/celo-blockchain/celo-blockchain/e2e_test/e2e_test.go:573 +0x56a
/opt/hostedtoolcache/go/1.17.5/x64/src/testing/testing.go:1259 +0x102
/opt/hostedtoolcache/go/1.17.5/x64/src/testing/testing.go:1306 +0x35a
```

I don't think it is caused by this PR, adding a nil check to make the
error case clearer can't hurt.

e2e_test/e2e_test.go

@@ -568,6 +568,7 @@ func TestEthersJSCompatibility(t *testing.T) {
 	err = network[0].Tracker.AwaitBlock(ctx, num+1)
 	require.NoError(t, err)
 	block := network[0].Tracker.GetProcessedBlock(num)
+	require.NotNil(t, block)
 
 	// Prune state
 	err = pruneStateOfBlock(ctx, network[0], block.Hash())