fix: move setting palette + json option setting to config

[kraftnix]

when no configuration is set (such as evaluating options for docs generation or LSP completions), a null value for image causes the options eval to fail

[trueNAHO]

this PR might let us remove the dummy values used for the documentation.

This is actually already possible: https://github.com/nix-community/stylix/pull/1318.