Fix walrus truthiness on metrics bounds and identity-partition projection

[kevinjqliu]

Inspired by the walrus issue in #3353

Summary

Several if x := dict.get(k): checks treated legitimate falsy values as missing:

• lower_bounds.get(field_id) / upper_bounds.get(field_id) return bytes. b"" is a valid serialization of an empty string and was being skipped, causing metrics-based row filtering to return ROWS_MIGHT_MATCH when it should have been ROWS_CANNOT_MATCH (and vice versa for strict eval).
• accessors[...].get(file.partition) can return 0 or "" for an IdentityTransform partition. The walrus dropped it, so projected partition columns were filled with null instead of the actual value.
• inspect.py lower_bound / upper_bound rendering had the same b"" issue, showing None instead of "" in readable_metrics.

All conditions are switched to explicit is not None checks. A small _readable_bound helper deduplicates the inspect rendering.

Changes

• pyiceberg/expressions/visitors.py — _InclusiveMetricsEvaluator and _StrictMetricsEvaluator bound lookups (21 sites).
• pyiceberg/io/pyarrow.py — _get_column_projection_values and ArrowProjectionVisitor missing-field handling.
• pyiceberg/table/inspect.py — extract _readable_bound, use it in both the entries and _get_files_from_manifest rendering paths.

Tests

• tests/expressions/test_evaluator.py — inclusive and strict evaluators with empty-string bounds, covering lower-only, upper-only, and both-bounds branches.
• tests/io/test_pyarrow.py — parametrized identity-transform projection with falsy partition values (0, "", None).
• tests/table/test_inspect.py — _readable_bound helper plus integration tests via tbl.inspect.entries() and tbl.inspect.files() for empty-string and null bounds.