moused,syscons: Avoid warning if system has no /dev/ttyv*

A VM had no virtual terminals and emitted a warning on boot
eval: cannot open /dev/ttyv*: No such file or directory.

Break the loop in this case to avoid the warning.

PR: 289173
Reviewed by: jlduran
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52344

(cherry picked from commit 5eb80bf1a6a435e88507c49111a28ed9e38b4b76)