Unsafe "None" checking

[MichaelMarien]

Throughout the library, a lot of initialisation check if an input is provided and compare it to the default value None. However usually this code looks like (example):

  def __init__(self,
                     rank: int,
                     ...
                     backend: Optional[Text] = None,
    ...
    if not backend:
      backend = `config.default_backend

Values like False,[], 0,... actually pass this test. A better way of testing is if backend is not None.