core: don't fail to run services in --user instances if $HOME is missing

Otherwise we cannot even invoke systemd-exit.service anymore, thus not
even exit.

https://bugs.freedesktop.org/show_bug.cgi?id=83100
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=759320

src/core/execute.c

@@ -1617,7 +1617,8 @@ static int exec_child(
                                 return -errno;
                         }
 
-                if (chdir(context->working_directory ? context->working_directory : "/") < 0) {
+                if (chdir(context->working_directory ?: "/") < 0 &&
+                    !context->working_directory_missing_ok) {
                         *exit_status = EXIT_CHDIR;
                         return -errno;
                 }