TLS is usually used on top of TCP and therefore UDP limits do not apply. I have no idea how implementations on top of UDP or other datagram protocols deal with

TLS is usually used on top of TCP and therefore UDP limits do not apply. I have no idea how implementations on top of UDP or other datagram protocols deal with that issue. For a payload length of zero I would expect to just get back a payload of length zero, but I am be no means a network or even TLS expert and the standard may as well disallow this. And I am to lazy to look it up. It is probably a really bad idea to make your protocol depend on details of an underlying protocol layer. Just imagine sending something over Ethernet and receiving it at the other end over WLAN where both protocols may disagree on the maximum packet size. You usually want your standard as precise as possible without imposing unnecessary and artificial constraints. Being allowed to stuff undefined amounts of data into a heart beat packet just to ease abusing the protocol really seems to call for trouble.