GH-3574: Preserve null counts when min/max exceed limit

[efegokdemir]

Summary

Preserve null_count when a column's min/max statistics exceed the metadata size limit.

The size guard is needed for min/max values, but null_count is a compact independent statistic and remains useful to readers. This keeps the null count available without reintroducing oversized min/max metadata.

Changes

• Emit null_count before applying the min/max size limit.
• Keep nan count and min/max omission behavior unchanged for oversized statistics.
• Extend the binary statistics round-trip test to cover the preserved null count.

Testing

• ./mvnw -pl parquet-hadoop -am -DskipITs -Dtest=TestParquetMetadataConverter -Dsurefire.failIfNoSpecifiedTests=false test
• ./mvnw -pl parquet-hadoop -am -DskipTests spotless:check
• git diff --check

All passed.

Notes

• No competing open PR for null_count is omitted for large columns in parquet files #3574 was present during the final duplicate check.
• AI assistance was used during development; the submitter reviewed the complete diff and is responsible for the change.

Fixes #3574

[divjotarora]

Thanks @efegokdemir!

[divjotarora]

@efegokdemir My previous comment was not addressed, the current code still sets NaN count stats after the withinLimit check. I suggest we set null + NaN stats unconditionally and guard only min/max by the withinLimit check.

Proposed code:

formatStats.setNull_count(stats.getNumNulls());
if (stats.isNanCountSet()) {
    formatStats.setNan_count(stats.getNanCount());
}

if (!withinLimit(stats, truncateLength)) {
    return formatStats;
}

[efegokdemir]

Moved nan_count alongside null_count before the withinLimit early return; min/max remain size-gated. Added testNanCountIsPreservedWhenMinMaxExceedLimit, which forces the floating-point stats size check over the limit and verifies null count, NaN count, and omitted min/max. The pushed remote HEAD is d015ab76dd7d2c9bf79f5795d971ee1c3a568f23, and I verified both changed files at that SHA. git diff --check passed. Focused Maven/Spotless validation could not run: the wrapper reports “Unable to locate a Java Runtime” in this environment.