[ETCM-980] encode empty "to" address in transaction RPC reply

[strauss-m]

Description

Transaction's field "to" was not encoded in contract creation since it was not set.
The facilitate ETS success, this field is expected to always be present.

Proposed Solution

The fix allows the encoding of empty fields, and remove the duplicated (and not working) custom option serializer

[jvdp]

Interesting, I actually did the same change way back but it was reverted quickly after: a8b92f5 (#965)

I wonder if you'll run into anything. Does retesteth not complain about extra fields?

[strauss-m]

Actually, unit-tests are not happy with this change.
eth_getTransactionReceipt is expecting either a root, or a status, and it's handled throught optional (see https://eth.wiki/json-rpc/API#eth_gettransactionreceipt).
I'm guess I should rollback the DefaultFormat.preserveEmptyValue part and do a hand-crafted custom json encoder for the EthTransactionResponse.