sys: util: Accept empty FOR_EACH

The ## part of ##__VA_ARGS__ in the Z_FOR_EACH_ENGINE macro technically
breaks the invocation of empty FOR_EACH sequences, as the empty
__VA_ARGS__ gets squashed with in the invocation of Z_FOR_LOOP_1() in
Z_FOR_LOOP_2(), which makes the macro only pass 4 arguments to
Z_FOR_LOOP_1.

This breaks IntelliSense in Microsoft's C/C++ extension for VS Code,
which is strict about the amount of arguments you can pass to a variadic
macro.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>
Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>