[hotfix][docs] Fix NullPointerException

Offset is Long type, the default value is null, and executing offset+=1 in Java 1.8.x will throw a NullPointerException

docs/dev/stream/state/state.md

@@ -640,7 +640,7 @@ public static class CounterSource
  implements ListCheckpointed<Long> {
 
  /** current offset for exactly once semantics */
- private Long offset;
+ private Long offset = 0L;
 
  /** flag for job cancellation */
  private volatile boolean isRunning = true;