fs/AllocatedPath: make the nullptr_t constructor public

src/storage/plugins/LocalStorage.cxx

@@ -124,7 +124,7 @@ LocalStorage::MapFS(const char *uri_utf8) const noexcept
 	try {
 		return MapFSOrThrow(uri_utf8);
 	} catch (...) {
-		return AllocatedPath::Null();
+		return nullptr;
 	}
 }