cpu-set-util: fix null pointer dereference and several cleanups (#37900)

Fixes CID#1611618.

diff --cc src/core/dbus-unit.c
Simple merge

diff --cc src/core/exec-invoke.c
Simple merge

diff --cc src/core/execute.c
Simple merge

diff --cc src/network/networkd-dhcp6.c
Simple merge

diff --cc src/shared/bus-unit-util.c
Simple merge

diff --cc src/test/test-cpu-set-util.c
Simple merge