Fix keyboard control for null step with marks

[weijiangan]

Currently, util.calculateNextValue() incorrectly returns mark object instead of value when steps are disabled (by passing null to step prop) and marks are used

The consequence of this is keyboard navigation will crash the slider due to unhandled errors that stem from unexpected value type.

Steps to reproduce

1. Pass a marks object as prop to slider component
2. Pass step = {null} as prop to slider component
3. Manipulate slider position using arrow keys

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.40%. Comparing base (34fca54) to head (4665779).
⚠️ Report is 218 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #607   +/-   ##
=======================================
  Coverage   95.40%   95.40%           
=======================================
  Files           2        2           
  Lines          87       87           
  Branches       29       29           
=======================================
  Hits           83       83           
  Misses          4        4           

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[yoyo837]

Rebase please.