There are absolutely no pre-requisites to taking the 819 exam.
You might find it pretty difficult, and want to ensure you are prepared, but there are no other certifications you need to have nor any other exams you have to have taken blocking you from registering for the 819.
Yes, this is a break from prior practice, where for a long while there were two different exams that needed to both be taken, the 819 and now the 829 break that tradition.
RTFJD (the JavaDocs are your friends!) If you haven't read them in a long time, then RRTFJD (they might have changed!)