palette: fix eval fail when image is null

[merrkry]

Fixes #911

Allows stylix.base16Scheme to be set to null when stylix.image is null. Therefore the assertion in the module can be triggered as expected.

Also adds lib.mkIf to this internal module, as the mentioned assertion will cause eval fail when the whole stylix module is not enabled. E.g. on large config with multiple hosts, where this module is not enabled on some devices.

[trueNAHO]

Cross-post:

Why is this not caught by the following assertion:

stylix/stylix/palette.nix

Lines 162 to 167 in 59d2c0e

	assertions = [
	{
	assertion = !(cfg.image == null && cfg.base16Scheme == null);
	message = "One of `stylix.image` or `stylix.base16Scheme` must be set";
	}
	];

base16Scheme has non-null default values. When nix tries to access its value in this assertion, the failure happens.

The cleanest solution might be to fix the assertion.

-- #911 (comment)

[merrkry]

Newer version:

• Wrap only the assertion with lib.mkIf as there are many use cases when stylix disabled
• Only assert stylix.base16Schme. As image is optional now, we only need to make sure a color scheme is generated (default value) or explicitly set.

[Flameopathic]

LGTM