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.

lib/neovim-plugin.nix

@@ -110,11 +110,12 @@ with lib;
           {
             extraPlugins = (optional installPackage cfg.package) ++ extraPlugins;
             inherit extraPackages;
-
-            ${extraConfigNamespace} = optionalString callSetup ''
+          }
+          (optionalAttrs callSetup {
+            ${extraConfigNamespace} = ''
               require('${luaName}')${setup}(${optionalString (cfg ? settings) (toLuaObject cfg.settings)})
             '';
-          }
+          })
           (optionalAttrs (isColorscheme && (colorscheme != null)) { colorscheme = mkDefault colorscheme; })
           (extraConfig cfg)
         ]);