Fixes handling of empty lists by Application

`Application.parse_preliminary_options_and_args` was previously, against
expectations, treating empty lists passed as the `argv` argument the
same way it treated `None`s.

This has been addressed and the correct behavior tested for in a unit
test of the `Application` class.

See issue #518 for details.

src/flake8/main/application.py

@@ -122,7 +122,7 @@ class Application(object):
         # do not need to worry and we can continue. If it is, we successfully
         # defer printing the version until just a little bit later.
         # Similarly we have to defer printing the help text until later.
-        args = (argv or sys.argv)[:]
+        args = (argv if argv is not None else sys.argv)[:]
         try:
             args.remove("--version")
         except ValueError: