sed: fix commandline-given expression when -e is not used

Make explicit sed commands (first on commandline) behave the same
as those given with -e.

Without this patch the following two commands behave differently,
the second one being wrong:
echo ab | sed -e $'1 i\\\n--'
echo ab | sed $'1 i\\\n--'

Reviewed by: 0mp, des, kevans
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D47377