[iceberg] Omit unknown null counts and bounds in IcebergDataFileMeta

[plusplusjiajia]

Purpose

IcebergDataFileMeta.create publishes wrong stats when a SimpleStats slot is
unknown (null), silently corrupting Iceberg's metrics-based pruning:

• an unknown null count was published as 0, so IS NULL pruning skips files
  that actually contain nulls;
• a required field's unknown min/max was read without a null check (the getter of
  a non-nullable type has none) and published as garbage bounds like [0, 0];
• bounds-ineligible types (nested, variant, vector, blob) were rejected only after
  reading the stats slots, which can crash on a required field.

Unknown counts and bounds are now omitted (a missing entry means unknown per the
Iceberg spec), the getter is built from the nullable copy of the type, and the
eligibility check runs before any slot is read. No format or API change.

[JingsongLi]

+1