lei up: infer rawstr from old searches via trailing "\n"

For --stdin searches created prior to commit 666dde69a3f6 (lei
q|up: fix saved searches for single-phrase search, 2021-11-08)
we still want to be able to run "lei up" on them without
regressions.  So assume nobody manages to enter "\n" as an
argv[] element and consider the presence of "\n" as a previous
--stdin use.

This fixes errors from "lei up" such as:

  lei_xsearch 2 wq_worker: Exception: Key too long: length was 840 bytes,
  maximum length of a key is 255 bytes at ../PublicInbox/IPC.pm line 250.

Fixes: 666dde69a3f6 ("lei q|up: fix saved searches for single-phrase search")