configtypes: Add none_ok param to UserStyleSheet.

qutebrowser/config/configdata.py

@@ -271,7 +271,7 @@ def data(readonly=False):
              "page."),
 
             ('user-stylesheet',
-             SettingValue(typ.UserStyleSheet(),
+             SettingValue(typ.UserStyleSheet(none_ok=True),
                           '::-webkit-scrollbar { width: 0px; height: 0px; }'),
              "User stylesheet to use (absolute filename, filename relative to "
              "the config directory or CSS string). Will expand environment "

qutebrowser/config/configtypes.py

@@ -1194,9 +1194,6 @@ class UserStyleSheet(File):
 
     typestr = 'user-stylesheet'
 
-    def __init__(self):
-        super().__init__(none_ok=True)
-
     def transform(self, value):
         if not value:
             return None