zephyr: Fix n=0 for utf8_lcpy

The function used an assert if n was 0. Instead
of using an assert, the function will now just
not do anything. The documentation has also
been updated to reflect this.

The reasoning for this is that the strlcpy function
this (sort of) implements for utf8 works the same way.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>