Check for color knob value existence before applying uppercase #6598
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Issue: #6597
What I did
Fixes #6597
How to test
https://github.com/storybooks/storybook/blob/c6d704225eec98c20a729242f14987852071ddb1/examples/official-storybook/stories/addon-knobs.stories.js#L65
["bug"]