The debug log for ClientHello message does not appear in JDK 12. The problem get exposed with the fix for JDK-8209965. It does not impact JDK 11 for now. But if backport JDK-8209965, please backport this fix as well.

diff -r 2a51125b2794 src/java.base/share/classes/sun/security/ssl/SSLExtensions.java
--- a/src/java.base/share/classes/sun/security/ssl/SSLExtensions.java Tue Sep 18 21:47:14 2018 -0700
+++ b/src/java.base/share/classes/sun/security/ssl/SSLExtensions.java Thu Sep 20 11:01:47 2018 -0700
@@ -333,7 +333,7 @@
             return "<no extension>";
         } else {
             StringBuilder builder = new StringBuilder(512);
- if (logMap != null) {
+ if (logMap != null && !logMap.isEmpty()) {
                 for (Map.Entry<Integer, byte[]> en : logMap.entrySet()) {
                     SSLExtension ext = SSLExtension.valueOf(
                             handshakeMessage.handshakeType(), en.getKey());