fix: return null in MinerGetBaseInfo if miner hasn't been around for 900 epochs

[lemmih]

Summary of changes

Changes introduced in this pull request:

• If a miner isn't found in the look-back state, we must return null to match Lotus' behavior.

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation. All new code adheres to the team's documentation standards,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

[LesnyRumcajs]

@lemmih In general, can we synthesize a test that would fail without this change? This is to ensure the behaviour doesn't change by chance in the future.

[lemmih]

Testing it using one of the devnets is possible in theory but rather tricky. It'll be much easier once we have #4657.

[lemmih]

Oof, more intermittent networking errors.

[LesnyRumcajs]

The CI just doesn't want this PR to be merged and finds plethora of excuses. :)