lib.generators.toINIWithGlobalSection: give sections a default

often sections aren't needed when using globalSection

currently one has to `sections = { }` when using only globalSection

```
lib.generators.toINIWithGlobalSection { } {
  globalSection = {
    background_color = "1d202199";
  };
  sections = { };
};
````

lib/generators.nix

@@ -168,7 +168,7 @@ rec {
     mkKeyValue    ? mkKeyValueDefault {} "=",
     # allow lists as values for duplicate keys
     listsAsDuplicateKeys ? false
-  }: { globalSection, sections }:
+  }: { globalSection, sections ? {} }:
     ( if globalSection == {}
       then ""
       else (toKeyValue { inherit mkKeyValue listsAsDuplicateKeys; } globalSection)