VARIANT value should be able to be omitted

[HuaHuaY]

Describe the bug, including details regarding any error messages, version, and platform.

There are some cases in parquet-testing https://github.com/apache/parquet-testing/tree/master/shredded_variant which don't have value field.

parquet-testing/shredded_variant on  master 
❯ ~/arrow/cpp/out/build/ninja-debug/debug/parquet-dump-schema case-041.parquet 
required group field_id=-1 table {
  required int32 field_id=1 id;
  optional group field_id=2 var (Variant(1)) {
    required binary field_id=-1 metadata;
    optional group field_id=-1 typed_value (List) {
      repeated group field_id=-1 list {
        required group field_id=-1 element {
          optional binary field_id=-1 value;
          optional binary field_id=-1 typed_value (String);
        }
      }
    }
  }
}

parquet-testing/shredded_variant on  master 
❯ ~/arrow/cpp/out/build/ninja-debug/debug/parquet-dump-schema case-131.parquet 
required group field_id=-1 table {
  required int32 field_id=1 id;
  required group field_id=2 var (Variant(1)) {
    required binary field_id=-1 metadata;
    optional int32 field_id=-1 typed_value;
  }
}

parquet-testing/shredded_variant on  master 
❯ ~/arrow/cpp/out/build/ninja-debug/debug/parquet-dump-schema case-132.parquet
required group field_id=-1 table {
  required int32 field_id=1 id;
  optional group field_id=2 var (Variant(1)) {
    required binary field_id=-1 metadata;
    optional binary field_id=-1 value;
    optional group field_id=-1 typed_value {
      required group field_id=-1 a {
        optional int32 field_id=-1 typed_value;
      }
      required group field_id=-1 b {
        optional binary field_id=-1 typed_value (String);
      }
    }
  }
}

parquet-testing/shredded_variant on  master 
❯ ~/arrow/cpp/out/build/ninja-debug/debug/parquet-dump-schema case-138.parquet 
required group field_id=-1 table {
  required int32 field_id=1 id;
  required group field_id=2 var (Variant(1)) {
    required binary field_id=-1 metadata;
    optional group field_id=-1 typed_value {
      required group field_id=-1 a {
        optional binary field_id=-1 value;
        optional int32 field_id=-1 typed_value;
      }
      required group field_id=-1 b {
        optional binary field_id=-1 value;
        optional binary field_id=-1 typed_value (String);
      }
    }
  }
}