IndicesRequestIT#testBulk fails with NullPointerException

[dnhatn]

This suite started failing since August 10th. May be related to #31821?

ERROR   0.57s J0 | IndicesRequestIT.testBulk <<< FAILURES!
   > Throwable #1: java.lang.NullPointerException
   > 	at org.elasticsearch.action.bulk.BulkItemRequest.index(BulkItemRequest.java:56)
   > 	at org.elasticsearch.action.bulk.BulkShardRequest.indices(BulkShardRequest.java:54)
   > 	at org.elasticsearch.action.IndicesRequestIT.assertIndicesSubset(IndicesRequestIT.java:626)
   > 	at org.elasticsearch.action.IndicesRequestIT.testBulk(IndicesRequestIT.java:315)
   > 	at java.lang.Thread.run(Thread.java:748)Throwable #2: java.lang.AssertionError: 
   > Expected: an empty iterable
   >      but: [<indices:data/write/bulk[s][r]=[ConcreteReplicaRequest{targetAllocationID='tPWE26HSR9yG-ukYI_I5AQ', primaryTerm='1', request=BulkShardRequest [[test0][0]] containing [4] requests, globalCheckpoint=-1}]>]
   > 	at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:20)
   > 	at org.elasticsearch.action.IndicesRequestIT.assertAllRequestsHaveBeenConsumed(IndicesRequestIT.java:686)
   > 	at org.elasticsearch.action.IndicesRequestIT.cleanUp(IndicesRequestIT.java:177)
   > 	at java.lang.Thread.run(Thread.java:748)

I can reproduce this locally around 10%.

./gradlew :server:integTest   
-Dtests.seed=3F68AD9A997FF672   
-Dtests.class=org.elasticsearch.action.IndicesRequestIT   
-Dtests.method="testBulk"   
-Dtests.security.manager=true  
-Dtests.locale=it-CH   
-Dtests.timezone=America/Dawson_Creek  
-Dcompiler.java=10   
-Druntime.java=8

Some instances:

• https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+6.x+intake/2248/console
• https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+master+multijob-unix-compatibility/os=oraclelinux/2616/console

[elasticmachine]

Pinging @elastic/es-distributed

[dnhatn]

I've muted this test on master and 6.x