modules/opts: add default value to g and opt

modules/vim/opts.nix

@@ -4,9 +4,11 @@
   options.vim = with lib; with types; {
     g = mkOption {
       type = attrsOf (oneOf [ str bool number ]);
+      default = { };
     };
     opt = mkOption {
       type = attrsOf anything;
+      default = { };
       description = ''
         A special interface |vim.opt| exists for conveniently interacting with list-
         and map-style option from Lua: It allows accessing them as Lua tables and