fix: support calling setup() w/o args

Avoids error `attempt to index local 'opts' (a nil value)` when calling
setup() without arguments.

Co-authored-by: Tristan Knight <tris203@gmail.com>

lua/precognition/init.lua

@@ -419,7 +419,8 @@ end
 
 ---@param opts Precognition.PartialConfig
 function M.setup(opts)
-    config = vim.tbl_deep_extend("force", default, opts or {})
+    opts = opts or {}
+    config = vim.tbl_deep_extend("force", default, opts)
     if opts.highlightColor then
         config.highlightColor = opts.highlightColor
     end