GH-3574: Keep null_count when min/max stats exceed the size limit

[arimu1]

Rationale for this change

When column statistics exceed the max size for truncation, toParquetStatistics currently omits the entire Statistics payload, including null_count. Dropping min/max is reasonable (there is no way to mark a truncated bound as a lower/upper bound rather than a true min/max). null_count and nan_count are tiny counters and should still be written.

Missing null_count causes downstream readers to treat the column as if it might contain nulls. Snowflake reports:

non-nullable column without default has null values according to file statistics

parquet-format recommends always writing null_count, even when it is zero.

This re-lands the parquet-java side of #3575 after it was reverted in #3688 to unblock a release. The format contract is that min/max and null counts are independent: Statistics.hasNonNullValue() vs Statistics.isNumNullsSet(). Consumers that assumed "all stats or no stats" should treat a missing min/max independently of a present null_count. Iceberg already observes num_nulls: N, min/max not defined on older files.

What changes are included in this PR?

In ParquetMetadataConverter.toParquetStatistics, write null_count (and nan_count when set) whenever stats are non-empty, even if min/max are omitted because they exceed MAX_STATS_SIZE. Min/max are still not written in that case (not truncated to an unmarked bound).

Are these changes tested?

Yes. TestParquetMetadataConverter.testBinaryStatsV1 / testBinaryStatsV2 now assert that oversized binary min/max are omitted while null_count is still written and round-trips.

./mvnw -pl parquet-hadoop -am test -Dtest=TestParquetMetadataConverter -Dsurefire.failIfNoSpecifiedTests=false

TestParquetMetadataConverter: 72 tests, 0 failures (Temurin 21, macOS aarch64).

On current master without this change, the same assertions fail (null_count expected 3004, actual 0).

Are there any user-facing changes?

Yes: files with oversized column min/max now still carry null_count (and nan_count when set) in column-chunk statistics. Readers that previously assumed the statistics object was all-or-nothing should use hasNonNullValue() / isSetMin_value() for bounds and isNumNullsSet() / isSetNull_count() for nulls.

Closes #3574

[arimu1]

Closing: this re-lands #3575, which was reverted in #3688 because Iceberg ParquetMetrics still treats Statistics.isEmpty() as all-or-nothing. Mixed row groups (small strings with min/max + large strings over MAX_STATS_SIZE) would then publish incomplete file-level bounds. Iceberg needs to keep null counts iff every chunk isNumNullsSet() and drop file-level bounds when a chunk has values but !hasNonNullValue(), before this parquet-java split is safe.

Sorry for the noise — #3574 remains valid once that consumer contract is evolved.