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

hugolib/config_test.go

@@ -152,6 +152,9 @@ name = "menu-top-main"
 	themeConfig := `
 baseURL = "http://bep.is/"
 
+# Can not be set in theme.
+disableKinds = ["taxonomy", "term"]
+
 # Can not be set in theme.
 [frontmatter]
 expiryDate = ["date"]
@@ -228,6 +231,9 @@ name = "menu-theme"
 
 		got := b.Cfg.Get("").(maps.Params)
 
+		// Issue #8866
+		b.Assert(b.Cfg.Get("disableKinds"), qt.IsNil)
+
 		b.Assert(got["params"], qt.DeepEquals, maps.Params{
 			"b": maps.Params{
 				"b1": "b1 main",