Fix the type of 'valid' attribute in LUDiagnoseOS

The update of the valid status in LUDiagnoseOS says:

  valid = valid and osl and osl[0][1]

However, in Python, “True and []” (which '[]' we get for an invalid OS)
will result in “[]”, and thus the valid field for an OS will be either
True or an empty list. Which is not what we want…

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>