Fix default autojump behavior when run with 0 arguments.

autojump

@@ -220,7 +220,7 @@ def shell_utility():
     """Run this when autojump is called as a shell utility"""
     parser = argparse.ArgumentParser(description='Automatically jump to directory passed as an argument.',
             epilog="Please see autojump(1) man pages for full documentation.")
-    parser.add_argument('directory', metavar='DIR', type=unicode, nargs='*', default=None,
+    parser.add_argument('directory', metavar='DIR', type=unicode, nargs='*', default=u'',
             help='directory to jump to')
     parser.add_argument('-a', '--add', metavar='DIR', type=unicode,
             help='add path to database')
@@ -261,7 +261,7 @@ def shell_utility():
     #default: gradually forget about old directories
     if (not args.completion): forget(path_dict, dic_file)
 
-    if (args.directory == None):
+    if (args.directory == u''):
         patterns = [unico("")]
     else:
         patterns = [decode(a) for a in args.directory]