Reduce memory footprint for nulls caching

Scenario:
There is a schema with many optional groups, e.g.
```
message example {
required binary id (UTF8);
optional group a1 (LIST) {
repeated int64 array;
}
optional group a2 (LIST) {
repeated int64 array;
}
...
optional group aN (LIST) {
repeated int64 array;
}
}
```

Many records without optional parameters are written.
In this case groupNullCache will contain many elements which all are zeros.

**Reporter**: [Boris Molodenkov](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=molodenkov)
#### Related issues:
- [Caching nulls on group node to improve write performance on wide schema sparse data](https://github.com/apache/parquet-java/issues/1860) (relates to)

_{**Note**: *This issue was originally created as [PARQUET-1119](https://issues.apache.org/jira/browse/PARQUET-1119). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

No source files or tests are named. Start by tracing where groupNullCache is created and populated, then read related issue #1860 for the existing caching design. Done means reducing storage for all-zero null-cache entries while preserving the relevant write behavior; add focused coverage where the current cache behavior is tested.

Written by the indexing model from the issue text.