Drop empty app data records in DTLS

Drop empty app data records in DTLS

App data records with 0 bytes of payload will confuse callers of SSL_read().
This will cause a successful read and return 0 bytes as read. Unfortunately
a 0 return from SSL_read() is considered a failure response. A subsequent
call to SSL_get_error() will then give the wrong result.

Zero length app data records are actually allowed by the spec, but have
never been handled correctly by OpenSSL. We already disallow creating such
empty app data records. Since the SSL_read() API does not have a good way to
handle this type of read, we simply ignore them.

Partial fix for #27316

Reviewed-by: Frederik Wedel-Heinen <fwh.openssl@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27541)