fix(admin): inconsistent return type on empty inputs

If an empty username or password are given during the initialization
of `emqx_dashboard_admin`, it'll return just the atom `ok`, which
breaks the expected `{ok, _}` in `emqx_dashboard_app:start/2`.

```
{'EXIT',
 {{failed_to_start_app,emqx_dashboard,
   {emqx_dashboard,
    {bad_return,
     {{emqx_dashboard_app,start,[normal,[]]},
      {'EXIT',
       {{badmatch,ok},
        [{emqx_dashboard_app,start,2,
          [{file,
            "/home/thales/dev/emqx/emqx/apps/emqx_dashboard/src/emqx_dashboard_app.erl"},
           {line,32}]},
         {application_master,start_it_old,4,
          [{file,"application_master.erl"},{line,293}]}]}}}}}}}}
```

apps/emqx_dashboard/src/emqx_dashboard_admin.erl

@@ -211,6 +211,7 @@ hash(Password) ->
 sha256(SaltBin, Password) ->
     crypto:hash('sha256', <<SaltBin/binary, Password/binary>>).
 
+-spec(add_default_user() -> {ok, map() | empty | default_user_exists } | {error, any()}).
 add_default_user() ->
     add_default_user(binenv(default_username), binenv(default_password)).
 
@@ -218,7 +219,7 @@ binenv(Key) ->
     iolist_to_binary(emqx_conf:get([emqx_dashboard, Key], "")).
 
 add_default_user(Username, Password) when ?EMPTY_KEY(Username) orelse ?EMPTY_KEY(Password) ->
-    ok;
+    {ok, empty};
 
 add_default_user(Username, Password) ->
     case lookup_user(Username) of