CI: DefaultShardsIT testDefaultShards fails with NPE in 6.x

[imotov]

Latest failure https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+6.x+intake/1925/console

> There were test failures: 5 suites, 930 tests, 1 error, 21 ignored (21 assumptions) [seed: 87E0F2D98E68DD1B]

REPRODUCE WITH: ./gradlew :distribution:archives:integ-test-zip:integTestRunner \
  -Dtests.seed=87E0F2D98E68DD1B \
  -Dtests.class=org.elasticsearch.test.rest.DefaultShardsIT \
  -Dtests.method="testDefaultShards" \
  -Dtests.security.manager=true \
  -Dtests.locale=en-GB \
  -Dtests.timezone=Asia/Hebron

java.lang.NullPointerException
	at __randomizedtesting.SeedInfo.seed([87E0F2D98E68DD1B:1534E97D1BBCF018]:0)
	at java.util.regex.Matcher.getTextLength(Matcher.java:1283)
	at java.util.regex.Matcher.reset(Matcher.java:309)
	at java.util.regex.Matcher.<init>(Matcher.java:229)
	at java.util.regex.Pattern.matcher(Pattern.java:1093)
	at org.elasticsearch.test.rest.DefaultShardsIT.testDefaultShards(DefaultShardsIT.java:36)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.carrotsearch.randomizedtesting.RandomizedRunner.invoke(RandomizedRunner.java:1713)
	at com.carrotsearch.randomizedtesting.RandomizedRunner$8.evaluate(RandomizedRunner.java:907)
	at com.carrotsearch.randomizedtesting.RandomizedRunner$9.evaluate(RandomizedRunner.java:943)
	at com.carrotsearch.randomizedtesting.RandomizedRunner$10.evaluate(RandomizedRunner.java:957)
	at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
	at org.apache.lucene.util.TestRuleSetupTeardownChained$1.evaluate(TestRuleSetupTeardownChained.java:49)
	at org.apache.lucene.util.AbstractBeforeAfterRule$1.evaluate(AbstractBeforeAfterRule.java:45)
	at org.apache.lucene.util.TestRuleThreadAndTestName$1.evaluate(TestRuleThreadAndTestName.java:48)
	at org.apache.lucene.util.TestRuleIgnoreAfterMaxFailures$1.evaluate(TestRuleIgnoreAfterMaxFailures.java:64)
	at org.apache.lucene.util.TestRuleMarkFailure$1.evaluate(TestRuleMarkFailure.java:47)
	at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
	at com.carrotsearch.randomizedtesting.ThreadLeakControl$StatementRunner.run(ThreadLeakControl.java:368)
	at com.carrotsearch.randomizedtesting.ThreadLeakControl.forkTimeoutingTask(ThreadLeakControl.java:817)
	at com.carrotsearch.randomizedtesting.ThreadLeakControl$3.evaluate(ThreadLeakControl.java:468)
	at com.carrotsearch.randomizedtesting.RandomizedRunner.runSingleTest(RandomizedRunner.java:916)
	at com.carrotsearch.randomizedtesting.RandomizedRunner$5.evaluate(RandomizedRunner.java:802)
	at com.carrotsearch.randomizedtesting.RandomizedRunner$6.evaluate(RandomizedRunner.java:852)
	at com.carrotsearch.randomizedtesting.RandomizedRunner$7.evaluate(RandomizedRunner.java:863)
	at org.apache.lucene.util.AbstractBeforeAfterRule$1.evaluate(AbstractBeforeAfterRule.java:45)
	at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
	at org.apache.lucene.util.TestRuleStoreClassName$1.evaluate(TestRuleStoreClassName.java:41)
	at com.carrotsearch.randomizedtesting.rules.NoShadowingOrOverridesOnMethodsRule$1.evaluate(NoShadowingOrOverridesOnMethodsRule.java:40)
	at com.carrotsearch.randomizedtesting.rules.NoShadowingOrOverridesOnMethodsRule$1.evaluate(NoShadowingOrOverridesOnMethodsRule.java:40)
	at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
	at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
	at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
	at org.apache.lucene.util.TestRuleAssertionsRequired$1.evaluate(TestRuleAssertionsRequired.java:53)
	at org.apache.lucene.util.TestRuleMarkFailure$1.evaluate(TestRuleMarkFailure.java:47)
	at org.apache.lucene.util.TestRuleIgnoreAfterMaxFailures$1.evaluate(TestRuleIgnoreAfterMaxFailures.java:64)
	at org.apache.lucene.util.TestRuleIgnoreTestSuites$1.evaluate(TestRuleIgnoreTestSuites.java:54)
	at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
	at com.carrotsearch.randomizedtesting.ThreadLeakControl$StatementRunner.run(ThreadLeakControl.java:368)
	at java.lang.Thread.run(Thread.java:748)

[elasticmachine]

Pinging @elastic/es-core-infra

[nik9000]

If I revert 78869eb then the test passes. @ywelsch wins!