fix(utils): add nil check for g:colors_name

[Jint-lzxy]

This commit is a follow-up fix for #1369 (and potentially #1371), simplifying the change to a single line:

--- /dev/fd/14	2025-01-18 15:44:55
+++ /dev/fd/15	2025-01-18 15:44:55
@@ -1 +1 @@
-palette = vim.g.colors_name:find("catppuccin") and require("catppuccin.palettes").get_palette()
+palette = (vim.g.colors_name or ""):find("catppuccin") and require("catppuccin.palettes").get_palette()