Permit sending empty fragments for TLS 1.0.

Due to a weakness in the TLS 1.0 protocol, OpenSSL will periodically
send empty TLS records ("empty fragments"). These TLS records have no
payload (and thus a page count of zero). m_uiotombuf_nomap() was
returning NULL instead of an empty mbuf, and a few places needed to be
updated to treat an empty TLS record as having a page count of "1" as
0 means "no work to do" (e.g. nothing to encrypt, or nothing to mark
ready via sbready()).

Reviewed by: gallatin
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D26729