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

Related issues:

• Caching nulls on group node to improve write performance on wide schema sparse data (relates to)

_{Note: This issue was originally created as PARQUET-1119. Please see the migration documentation for further details.}

未指定源文件或测试。首先跟踪 groupNullCache 的创建和填充位置，然后阅读关于现有缓存设计的 issue #1860。完成标准是在保留相关写入行为的同时，减少全零 null-cache 条目的存储；在测试当前缓存行为的位置添加有针对性的覆盖。