Keyboard navigations for uneven marks when the step is set to `null` is not working as expected

[Humaira-Sultana]

The keyboard navigations are not working as expected when the step value is set to null. This can be reproduced in the official website example.

Steps to reproduce the issue:

1. In the above-mentioned example, click on the mark to get the focus on the slider.
2. Now click on the right/left arrow and observe the navigations.
3. When the right arrow is clicked the slider moves to the left for 1 value and then stops working.

Please look into this.