Fixes the adapter of empty nested responses. #526

Job	Run time
CMake msvc-14.2 Debug C++17	6m 41s
CMake gcc-11 17 Debug	8m 50s
CMake msvc-14.2 Release C++17	8m 42s
B2 msvc-14.2	7m 12s
CMake gcc-11 20 Release	11m 9s
B2 gcc-11	4m 40s
B2 msvc-14.3	6m 54s
CMake msvc-14.3 Debug C++20	7m 10s
CMake clang-11 17 Debug	6m 23s
B2 clang-14	5m 30s
CMake msvc-14.3 Release C++20	7m 30s
CMake clang-11 20 Debug	6m 33s
CMake clang-13 17 Release	9m 54s
CMake clang-13 20 Release	9m 53s
CMake clang-14 17 Debug -stdlib=libc++	8m 10s
CMake clang-14 20 Release -stdlib=libc++	12m 9s
	2h 7m 20s

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