ConcurrentModificationException in ParDoSchemaTest

When ran with Java 11, ParDoSchemaTest.testReadAndWriteMultiOutput,
testReadAndWrite and testReadAndWriteWithSchemaRegistry

produce ConcurrentModificationExceptions.

This is due to change in HashMap.computeIfAbsent method specification:

Throws:ConcurrentModificationException - if it is detected that the mapping function modified this map

Full documentation here