Made admin password field required on client side

The admin login field is required on client side, but the admin password field is required only on server side. If empty password provided then server returns error and red warning. This change checks for empty password on client side (the same way it's done for login).

Fix for case described in syncloud/owncloud-setup#19

MIT licensed

The same commit is pull requested to master as well: https://github.com/owncloud/core/pull/6561