[Sort] If there is no direction, active should not be populated with a sortable ID

[andrewseguin]

After cycling through the sort options, if a sortable's direction becomes empty ('') then MatSort's this.active should revert to an empty string or undefined again.

[andrewseguin]

The active property has this declaration:

  /** The id of the most recently sorted MatSortable. */
  @Input('matSortActive') active: string;

I think we should change this to be an empty string when there is no direction and thus imply there is no active sortable. Keeping this set to the most recent sortable even if its not sorted is misleading with a name like active

[andrewseguin]

Closing because changing this will be too disruptive and breaking for existing applications, which have built their logic on this behavior.