add zero-length BEAST mitigation and Java caveat

This adds the detection of zero-length application data records for BEAST attack mitigation (sometimes used by OpenSSL-based tools instead of TLS 1.2 or 1.1). It also adds a caveat about Java's style of record splitting but doesn't go into details about it. I kind of want to go into detail about it, but the about page is probably not the right venue.