…oat from blank fields

Noticed while debugging Tharsis/EVMOS logs that the logs were
extraneous with empty fields such as
```json
{"pc":4716,"op":80,"gas":"0x688e5","gasCost":"0x2","memory":"0x","memSize":352,"stack":["0xfa31de01","0x29a","0x7d0","0xabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcd","0x80","0x534","0xc0","0x5f646b5592c4159d4963fbf901f81144ef53cc51a7082435e3a6120a45002ff7","0x15dc","0x34","0x5f646b5592c4159d4963fbf901f81144ef53cc51a7082435e3a6120a45002ff7"],"returnData":"0x","depth":2,"refund":0,"opName":"POP","error":""}
{"pc":4717,"op":80,"gas":"0x688e3","gasCost":"0x2","memory":"0x","memSize":352,"stack":["0xfa31de01","0x29a","0x7d0","0xabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcd","0x80","0x534","0xc0","0x5f646b5592c4159d4963fbf901f81144ef53cc51a7082435e3a6120a45002ff7","0x15dc","0x34"],"returnData":"0x","depth":2,"refund":0,"opName":"POP","error":""}
```

with empty fields like "error", "refund", "memory", "returnData".

This change adds the JSON tag ",omitempty" to remove the bloat from the
above empty fields which shows immediate impact of reducing the logs to

```json
{"pc":4716,"op":80,"gas":"0x688e5","gasCost":"0x2","memSize":352,"stack":["0xfa31de01","0x29a","0x7d0","0xabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcd","0x80","0x534","0xc0","0x5f646b5592c4159d4963fbf901f81144ef53cc51a7082435e3a6120a45002ff7","0x15dc","0x34","0x5f646b5592c4159d4963fbf901f81144ef53cc51a7082435e3a6120a45002ff7"],"depth":2,"opName":"POP"}
{"pc":4717,"op":80,"gas":"0x688e3","gasCost":"0x2","memSize":352,"stack":["0xfa31de01","0x29a","0x7d0","0xabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabcd","0x80","0x534","0xc0","0x5f646b5592c4159d4963fbf901f81144ef53cc51a7082435e3a6120a45002ff7","0x15dc","0x34"],"depth":2,"opName":"POP"}
```

and not only does that reduce the size of logs but it also makes it much
easier to quickly debug and grep for errors, which will only be present if
erroring.

Fixes ethereum#24487