fix(snack-bar): snack bar not animating in if no positions are passed in

[crisbeto]

Adds some better handling for the case where no positions are passed into a snack bar. Currently the snack bar attempts to animate in to an invalid animation state.

Fixes #11197.

[josephperrott]

I think a better way to tackle this would be to set ensure the defaults are set in the open method in snack-bar.ts

We can do:

open(message: string, action: string = '', config?: MatSnackBarConfig):
    MatSnackBarRef<SimpleSnackBar> {
  const _config = {...new MatSnackBarConfig(), ...this._defaultConfig, ...config};

  // Since the user doesn't have access to the component, we can
  // override the data to pass in our own message and action.
  _config.data = {message, action};
  _config.announcementMessage = message;

  return this.openFromComponent(SimpleSnackBar, _config);
}

This will have our object as the default, while being overridden by whatever the user provides either by provider or at the time open is called.

[crisbeto]

That's what I was thinking initially. My reasoning for doing it this way is that the purpose of using the injection token in the first place is to be able to change the default values, but by creating a new MatSnackBarConfig, you're already setting some defaults that come with the class. That being said, we can go with the simpler solution and see if people end up being confused about it.

[jelbourn]

I see specifying the default options as overriding the defaults for the ones you provide; anything left unspecified would just be the original default.

[crisbeto]

The feedback has been addressed.

[josephperrott]

LGTM

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