Since `AssetCatalogPath` isn't default constructible (unlike the
previous `CatalogPath`, alias `std::string`), `AssetCatalog` isn't
default constructible either. But its default constructor is declared
with `= default` which Apple Clang was warning about.
Differential Revision: https://developer.blender.org/D12714
Reviewed by: Sybren Stüvel