modules: avoid setting empty strings to extraConfig* options

Problem:  Some modules are setting empty strings to extraConfig* options
          with the intention to not generate any config. But empty
          strings are also values, so they are still concatenated in the
          final value of extraConfig* options. This results in a
          multiple empty strings in extraConfigs.

Solution: Avoid using optionalString when setting values to extraConfig*
          options. Use mkIf instead.

          This commit also fixes mkIf condition in autocmd module.

          `mkNeovimPlugin` is a special case. To avoid evaluating
          caller's arguments mkMerge/optionalAttrs pattern is used
          instead.

modules/diagnostics.nix

@@ -19,7 +19,7 @@ with lib;
   };
 
   config = {
-    extraConfigLuaPre = optionalString (config.diagnostics != { }) ''
+    extraConfigLuaPre = mkIf (config.diagnostics != { }) ''
       vim.diagnostic.config(${helpers.toLuaObject config.diagnostics})
     '';
   };