The just released Java 13 early access build 20, shows the following output for "java -version" command:

openjdk version "13-null" 2019-09-17
OpenJDK Runtime Environment (build 13-null+20)
OpenJDK 64-Bit Server VM (build 13-null+20, mixed mode, sharing)

This shows up on Linux as well as macos builds downloaded from http://jdk.java.net/13/

Furthermore, some of the related Java System properties too contain "null" as part of their value strings. For example:

java.runtime.version=13-null+20
java.version=13-null
java.vm.version=13-null+20

P.S: I've marked this as "Affects version" 13 but I am not sure if that's the right version to choose.