net: lwm2m: bugfix lwm2m shell exec command with optional parameter

Actually `execute_cb` is fed with an array of char* and the size of
that array, instead of a single char* buffer and its byte size.

This fix expects a single, already joined, optional shell argument and
feeds `execute_cb` with proper arguments.

Signed-off-by: Stefan Schwendeler <Stefan.Schwendeler@husqvarnagroup.com>