libpromises/evalfunction: Do not crash w/o arguments #1637

Job	Run time
shellcheck_tests / Run shellcheck on shell scripts	1m 5s
unit_tests / Run Unit Tests	2m 31s
macos_unit_tests / macos_unit_tests	2m 50s
windows_acceptance_tests / windows_acceptance_tests	16m 22s
asan_unit_tests / asan_unit_tests	2m 36s
acceptance_tests / acceptance_tests	31m 51s
static_check / static_check	8m 51s
valgrind_tests / valgrind_tests	6m 5s
valgrind_check / valgrind_check	6m 1s
	1h 18m 12s

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