Atomicity violations caused by improper usage of ConcurrentHashMap

Code: https://github.com/apache/kafka/blob/trunk/connect/runtime/src/main/java/org/apache/kafka/connect/runtime/WorkerConfigTransformer.java#L81-L84 

Problem
We are using a ConcurrentHashMap, but usage of get(), null check, and put() may not be thread-safe at lines: 81, 82, and 84. Two threads can perform this same check at the same time and one thread can overwrite the value written by the other thread.

Fix
Consider replacing put() with putIfAbsent() to help prevent accidental overwriting. putIfAbsent() puts the value only if the ConcurrentHashMap does not contain the key and therefore avoids overwriting the value written there by the other thread's putIfAbsent().