Fix build when nlohmann is not detected (reported by cdash ci on wind… #2300

Job	Run time
build-ubuntu-dep-apt (ubuntu-20.04, /usr/bin/gcc-9, /usr/bin/g++-9, 98)	8m 25s
build-ubuntu-dep-apt (ubuntu-20.04, /usr/bin/gcc-9, /usr/bin/g++-9, 11)	25m 29s
build-ubuntu-dep-apt (ubuntu-20.04, /usr/bin/gcc-9, /usr/bin/g++-9, 17)	30m 10s
build-ubuntu-dep-apt (ubuntu-20.04, /usr/bin/gcc-10, /usr/bin/g++-10, 98)	7m 27s
build-ubuntu-dep-apt (ubuntu-20.04, /usr/bin/gcc-10, /usr/bin/g++-10, 11)	22m 50s
build-ubuntu-dep-apt (ubuntu-20.04, /usr/bin/gcc-10, /usr/bin/g++-10, 17)	26m 47s
build-ubuntu-dep-apt (ubuntu-20.04, /usr/bin/clang, /usr/bin/clang++, 98)	9m 4s
build-ubuntu-dep-apt (ubuntu-20.04, /usr/bin/clang, /usr/bin/clang++, 11)	30m 32s
build-ubuntu-dep-apt (ubuntu-20.04, /usr/bin/clang, /usr/bin/clang++, 17)	34m 0s
build-ubuntu-dep-apt (ubuntu-latest, /usr/bin/gcc-9, /usr/bin/g++-9, 98)	6m 40s
build-ubuntu-dep-apt (ubuntu-latest, /usr/bin/gcc-9, /usr/bin/g++-9, 11)	22m 23s
build-ubuntu-dep-apt (ubuntu-latest, /usr/bin/gcc-9, /usr/bin/g++-9, 17)	25m 7s
build-ubuntu-dep-apt (ubuntu-latest, /usr/bin/gcc-10, /usr/bin/g++-10, 98)	7m 42s
build-ubuntu-dep-apt (ubuntu-latest, /usr/bin/gcc-10, /usr/bin/g++-10, 11)	22m 19s
build-ubuntu-dep-apt (ubuntu-latest, /usr/bin/gcc-10, /usr/bin/g++-10, 17)	28m 8s
build-ubuntu-dep-apt (ubuntu-latest, /usr/bin/clang, /usr/bin/clang++, 98)	9m 54s
build-ubuntu-dep-apt (ubuntu-latest, /usr/bin/clang, /usr/bin/clang++, 11)	32m 18s
build-ubuntu-dep-apt (ubuntu-latest, /usr/bin/clang, /usr/bin/clang++, 17)	37m 33s
	6h 26m 48s

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