text: Title: Using bare `Union` as annotation is not handled properly
  Body: ```py
  from typing import Union
  def foo(a: Union): ...
  ```
  ```robot
  Tc
      foo    1
  ```
  
  ```
  👉 python -m robot.run test.robot
  ==============================================================================
  Test                                                                          
  ==============================================================================
  Tc                                                                    | FAIL |
  AttributeError: __args__
  ------------------------------------------------------------------------------
  ```