MatTableDataSource.sortingDataAccessor needs sortDirection

#### Bug, feature request, or proposal:

Feature request

Enhance MatTableDataSource.sortingDataAccessor with sortDirection parameter to allow for better sort customization.

#### What is the expected behavior?

For example being able to define a sortingDataAccessor to *always* sort NULL values to the bottom (or top for that matter) regardless if 'asc' or 'desc'

#### What is the current behavior?

Null values are interpreted as 0 (zero) in case of numeric properties.

#### What are the steps to reproduce?

https://angular-8ubc1i.stackblitz.io

#### What is the use-case or motivation for changing an existing behavior?

I have a list of objects with various numeric properties, some of which are NULL.

I'd like to write a sortingDataAccessor to ALWAYS sort NULL values to the bottom regardless of 'asc' or 'desc' (since NULL is irrelevant/meaningless for our sort)
For this I seem to need a sortDirection in the sortingDataAccessor to either return min or max 'infinity' for NULL values.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?

6.4.7

#### Is there anything else we should know?

MatTableDataSource とその sortingDataAccessor の実装を見つけ、リンク先の StackBlitz の例で動作を再現してください。提案する accessor がソート方向を使用でき、昇順と降順のどちらでも NULL 値が一貫して末尾または先頭に残ることを確認してください。

索引モデルが issue の本文から書いたものです。