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

[cracauer]

Summary

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--'

Sponsored by: Klara, Inc.