Asset Catalogs: allow creating catalog in unsaved blend file

Allow creating a new asset catalog in a yet-to-be-saved blend file. The
problem was caused by `AssetLibrary` not having an `AssetCatalogService`
right after creation; only after loading data from disk was this
instance created. It's now always there.

source/blender/blenkernel/BKE_asset_library.hh

@@ -40,6 +40,7 @@ namespace blender::bke {
 struct AssetLibrary {
   std::unique_ptr<AssetCatalogService> catalog_service;
 
+  AssetLibrary();
   ~AssetLibrary();
 
   void load(StringRefNull library_root_directory);

source/blender/blenkernel/intern/asset_library.cc

@@ -102,6 +102,10 @@ void BKE_asset_library_refresh_catalog_simplename(struct AssetLibrary *asset_lib
 
 namespace blender::bke {
 
+AssetLibrary::AssetLibrary() : catalog_service(std::make_unique<AssetCatalogService>())
+{
+}
+
 AssetLibrary::~AssetLibrary()
 {
   if (on_save_callback_store_.func) {