Fix it so disableKinds etc. does not get merged in from theme

Unless the merge strategy is set up to do so.

For `disableKinds` the current workaround is to make sure the project config has an entry, even if is empty:

```
disableKinds = []
```

Note that this issue only touches root, non-map config-values that either is not set in project config or in Hugo's defaults.

Fixes #8866

config/defaultConfigProvider_test.go

@@ -172,7 +172,6 @@ func TestDefaultConfigProvider(t *testing.T) {
 
 		c.Assert(cfg.Get(""), qt.DeepEquals, maps.Params{
 			"a": "av",
-			"b": "bv2",
 		})
 	})