I guess the fact that a three line function needed explaining hints that it perhaps could be better named. How about this?
```
def extract_bearer_token(auth_header):
    """
    >>> extract_bearer_token('Bearer some-token')
    u'some-token' 
    >>> extract_bearer_token('Bearer ')          
    None         
    >>> extract_bearer_token('Something Else')           
    None         
    """
    prefix = 'Bearer '
    if not auth_header is None or not auth_header.startswith(prefix):                  return None
    token = auth_header[len(prefix):]           
    return token if len(token) else None
```