The RAW encoder of the charstring & universal charstring was modified to
handle null terminated encoding.

New FIELDLENGTH value was introduced: null_terminated

example:
type charstring null_str with {variant "FIELDLENGTH(null_terminated)"}

If the string is null terminated, the fieldlength filed of the raw
descriptor is set to -8

Signed-off-by: Gabor Szalai <gabor.szalai@ericsson.com>