Ninja Multi-Config: hard crash when genex leaves add_custom_command OUTPUT empty

Example:

cmake_minimum_required(VERSION 3.20)
project(test NONE)

add_custom_command(OUTPUT $<$<CONFIG:Debug>:debug.txt>
                   COMMAND echo foo > debug.txt)

add_custom_target(genFiles ALL DEPENDS "$<$<CONFIG:Debug>:${CMAKE_CURRENT_BINARY_DIR}/debug.txt>")

At the console:

D:\output-gen>cmake -G "Ninja Multi-Config" -S . -B build
-- Configuring done

D:\output-gen>dir build
 Volume in drive D is Sources
 Volume Serial Number is 943F-828D

 Directory of D:\output-gen\build

03/25/2021  04:22 PM    <DIR>          .
03/25/2021  04:22 PM    <DIR>          ..
03/25/2021  04:22 PM                 0 build-Debug.ninja.tmp601a3
03/25/2021  04:22 PM                 0 build-Release.ninja.tmp40cb0
03/25/2021  04:22 PM                 0 build-RelWithDebInfo.ninja.tmp379e5
03/25/2021  04:22 PM               471 build.ninja
03/25/2021  04:22 PM                 0 build.ninja.tmpce904
03/25/2021  04:22 PM             4,135 CMakeCache.txt
03/25/2021  04:22 PM    <DIR>          CMakeFiles
03/25/2021  04:22 PM             1,407 cmake_install.cmake
               7 File(s)          6,013 bytes
               3 Dir(s)  415,645,818,880 bytes free

Discovered while trying to fix #21983 (closed)