Remove create_empty_fragment from do_dtls1_write()

Remove create_empty_fragment from do_dtls1_write()

do_dtls1_write() was never called with a value for create_empty_fragment
that was ever non-zero - so this is dead code and can be removed. The
equivalent code in the TLS processing is used for TLS1.0/SSLv3 to protect
against known IV weaknesses because those protocol versions do not have
an explicit IV. However DTLS1.0 is based on TLSv1.1 and *does* have an
explicit IV - so this is not useful there.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19424)