Cannot set slider value back to undefined

[ClivePixton]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

If a slider has some numeric value, and then the value is set to 'undefined', I would expect the slider to move to the minimum value position.

What is the current behavior?

If a slider has some numeric value, and then the value is set to 'undefined', the slider position does not change.

What are the steps to reproduce?

https://angular-material2-issue-wlz1aj.stackblitz.io

(Note that the slide toggle does behave as expected)

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

I would like to be able to restore a set of sliders to a previous state (i.e. when undoing), however if the state includes 'undefined' values, that state cannot be accurately restore.

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

Angular: 5.2.2
Material: 5.1.1
OS: darwin x64
TypeScript: 2.6.2
Browser: Chrome 63.0.3239.132

Is there anything else we should know?

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}