tls: more permissive empty data eof check

tls: more permissive empty data eof check

If not all data is ACK'd during the FIN session shutdown, the last calls
to the parser can be with a non-NULL data pointer, but a input length of
0. This wasn't considered by the EOF check, which then lead to it being
seen as an error. No event was raised, but the tls error stats were
incremented.

Bug: #7554.