Because of "if x else ''" in _decode_args (http://hg.python.org/cpython/file/3.4/Lib/urllib/parse.py#l96), urllib.parse.urlparse accepts any falsy value as an url, returning a ParseResultBytes with all members set to empty bytestrings.
Thus you get:
>>> urllib.parse.urlparse({})
ParseResultBytes(scheme=b'', netloc=b'', path=b'', params=b'', query=b'', fragment=b'')
which may result in some very confusing exceptions later on: I had a list of URLs that accidentally contained some Nones and got very confusing TypeErrors while processing the results expecting them to be strings.
If the `if x else ''` part were removed, such invalid falsy values would fail with `AttributeError: 'foo' object has no attribute 'decode'`, as happens with any truthy invalid value.