Encode zero-length hex-strings with quotes

[jtraglia]

When running through the reference tests, I noticed that zero-length hex-strings are encoded without quotation marks. For example, in this test case which has a zero-length extra data field:
https://media.githubusercontent.com/media/ethereum/consensus-spec-tests/master/tests/mainnet/electra/ssz_static/ExecutionPayloadHeader/ssz_random/case_1/value.yaml

In other yaml files, this has quotation marks:

My solution was to wrap the yaml string representer method with a special case for hex-strings.

[ralexstokes]

the change seems fine as long as it doesn't demand a lot of code change from existing consumers (i.e. client teams)

I expect they had to special case on their end already to handle this deviation and so this would be a simplification even if it does require touching some of the test-parsing code