storage/{composite,local}: fix -Wnonnull warnings

src/storage/CompositeStorage.cxx

@@ -332,7 +332,7 @@ CompositeStorage::MapToRelativeUTF8(std::string_view uri) const noexcept
 	}
 
 	if (!root.MapToRelativeUTF8(relative_buffer, uri))
-		return nullptr;
+		return {};
 
 	return relative_buffer;
 }

src/storage/plugins/LocalStorage.cxx

@@ -106,8 +106,6 @@ LocalStorage::MapUTF8(std::string_view uri_utf8) const noexcept
 AllocatedPath
 LocalStorage::MapFSOrThrow(std::string_view uri_utf8) const
 {
-	assert(uri_utf8 != nullptr);
-
 	if (uri_utf8.empty())
 		return base_fs;