text: Title: Libdoc crashes if it does not detect documentation format
  Body: If documentation format isn't detected automatically based on the output file extension or given explicitly using `--format`, Libdoc execution fails with `AttributeError`:
  ```
  $ libdoc BuiltIn BuiltIn
  Unexpected error: AttributeError: 'NoneType' object has no attribute 'upper'
  Traceback (most recent call last):
    File "/home/peke/Devel/robotframework/src/robot/utils/application.py", line 81, in _execute
      rc = self.main(arguments, **options)
    File "src/robot/libdoc.py", line 197, in main
      libdoc.save(output, format, self._validate_theme(theme, format))
    File "/home/peke/Devel/robotframework/src/robot/libdocpkg/model.py", line 93, in save
      with LibdocOutput(output, format) as outfile:
    File "/home/peke/Devel/robotframework/src/robot/libdocpkg/output.py", line 27, in __init__
      self._format = format.upper()
  AttributeError: 'NoneType' object has no attribute 'upper'
  ```
  
  This is an error situation, but it should be reported correctly.