core/types: test invalid log JSON data

[Amlan2006]

Adds four cases to TestUnmarshalLog to verify rejection of invalid data fields:

• Explicit null.
• Odd-length hex ("0x0").
• Invalid hex characters ("0xzz").
• Missing 0x prefix ("1234").

Each case checks the expected decoding error. No production code changes.

Validation:

• Passed: go test ./core/types -run '^TestUnmarshalLog$' -count=1

[rjl493456442]

It's more the tests for the JSON unmarshaller itself. I don't think it's necessary