text: Title: `List Should Contain Sub List` does not detect failure if only difference is empty string
  Body: I expect all of the following to raise AssertionError, as the second list is not a subset of the first:
  
  ```python
  from robot.libraries.Collections import Collections
  
  second = [""]
  Collections().list_should_contain_sub_list(["first"], second)
  
  
  second = ["first", ""]
  Collections().list_should_contain_sub_list(["first"], second)
  
  
  second = ["", "first"]
  Collections().list_should_contain_sub_list(["first"], second)
  
  
  second = ["", ""]
  Collections().list_should_contain_sub_list(["first"], second)
  ```
  
  However, only the last one raises the error.