utils: silence mypy on pa.nulls() abstract-DataType overload

[rustyconover]

Summary

Unblocks the CI Lint job, which has been failing on every PR (including main) since pyarrow's stubs tightened the `nulls()` overload set.

The stubs declare one overload per concrete `DataType` subtype (`Int8Type`, `StringType`, ...) but no overload for the abstract `DataType` base. `_empty_array` accepts `pa.DataType` because callers pass arbitrary inferred Arrow types; the runtime behaviour is correct for every concrete subtype, so the call is safe. The two ignores silence `call-overload` + `no-any-return` on the one line.

Test plan

• `uv run ruff format --check .` / `ruff check .` → clean
• `uv run mypy vgi_rpc/ --any-exprs-report ...` → Success: no issues found in 60 source files (was failing with 2 errors before)
• Type precision: 97.18% (well above the 95% CI threshold)
• `uv run pytest --timeout=50` → 3324 passed

🤖 Generated with Claude Code

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