Fix HeaderDict with an empty value

qutebrowser/config/configtypes.py

@@ -1475,6 +1475,8 @@ class HeaderDict(BaseType):
 
 
     def transform(self, value):
+        if not value:
+            return None
         val = json.loads(value)
         return val or None
 

tests/unit/config/test_configtypes.py

@@ -1745,6 +1745,7 @@ class TestHeaderDict:
     @pytest.mark.parametrize('val, expected', [
         ('{"foo": "bar"}', {"foo": "bar"}),
         ('{}', None),
+        ('', None),
     ])
     def test_transform(self, klass, val, expected):
         assert klass(none_ok=True).transform(val) == expected