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

Merging #607 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #607   +/-   ##
======================================
  Coverage    95.4%   95.4%           
======================================
  Files           2       2           
  Lines          87      87           
  Branches       29      29           
======================================
  Hits           83      83           
  Misses          4       4

Impacted Files	Coverage Δ
src/utils.js	95% <100%> (ø)	⬆️

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[yoyo837]

Rebase please.