chore: fix serialize/deserialize for `NodeUpdate` and `NodeCreate`

[0xivanov]

Description:

This PR fixes:

1. NodeCreate and NodeUpdate have nullable fields that cannot be set to null.
2. NodeUpdateTransaction gossip and grpc certificate bytes deserialize to empty ByteValue/ByteString

Related issue(s):

Fixes #2090 #2091

Notes for reviewer:

Checklist

• [x] Documented (Code comments, README, etc.)
• [x] Tested (unit, integration, etc.)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 82.72%. Comparing base (b86ee46) to head (a5faab7). Report is 159 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2092 +/- ## ============================================ + Coverage 82.20% 82.72% +0.52% - Complexity 3588 3944 +356 ============================================ Files 186 199 +13 Lines 11697 12725 +1028 Branches 1150 1257 +107 ============================================ + Hits 9615 10527 +912 - Misses 1604 1712 +108 - Partials 478 486 +8 ```

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