kubecolor: omit preset instead of declaring empty string

[trueNAHO]

Fixes: 74ee1ed5057e ("kubecolor: init (#657)")

@ajgon, could you verify whether this is semantically equivalent and works in all cases? If lib.mkIf is unsupported in this context, lib.optionalString could be used instead.

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[ajgon]

@trueNAHO checked and verified, lib.mkIf is fine in this case - for either it will drop the preset line, which kubecolor handles without any issue.