FAPI: Fix segfault if json field is null. #2184

Job	Run time
build-test (ubuntu-20.04, gcc)	4m 27s
build-test (ubuntu-20.04, clang)	4m 21s
build-test (fedora-32, gcc)	4m 39s
scanbuild	6m 35s
build-test (fedora-32, clang)	4m 59s
build-test (opensuse-leap, gcc)	4m 30s
build-test (opensuse-leap, clang)	4m 31s
build-test (ubuntu-22.04, gcc)	4m 20s
build-test (ubuntu-22.04, clang)	4m 39s
build-test (alpine-3.15, gcc)	4m 5s
build-test (alpine-3.15, clang)	4m 52s
test-coverage	5m 26s
test-fuzz	2m 27s
test-mbedtls (ubuntu-20.04)	2m 43s
test-no-crypto-build	1m 22s
test-mbedtls (ubuntu-22.04-mbedtls-3.1)	2m 36s
test-tcti-config	14m 10s
coverity-test	0s
	1h 20m 42s

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