Protects against NPE:

2> REPRODUCE WITH: ./gradlew ':x-pack:plugin:watcher:test' --tests "org.elasticsearch.xpack.watcher.history.HistoryTemplateTransformMappingsTests.testTransformFields" -Dtests.seed=26754396AB9C1A30 -Dtests.security.manager=true -Dtests.locale=lv-LV -Dtests.timezone=America/Dominica -Dcompiler.java=13 -Druntime.java=8
  2> java.lang.NullPointerException
        at __randomizedtesting.SeedInfo.seed([26754396AB9C1A30:B2A3CA27E260803B]:0)
        at org.elasticsearch.xpack.watcher.history.HistoryTemplateTransformMappingsTests.lambda$testTransformFields$1(HistoryTemplateTransformMappingsTests.java:85)
        at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
        at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
        at java.util.HashMap$ValueSpliterator.forEachRemaining(HashMap.java:1628)
        at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
        at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
        at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
        at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
        at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
        at org.elasticsearch.xpack.watcher.history.HistoryTemplateTransformMappingsTests.lambda$testTransformFields$2(HistoryTemplateTransformMappingsTests.java:88)
        at org.elasticsearch.test.ESTestCase.assertBusy(ESTestCase.java:892)
        at org.elasticsearch.test.ESTestCase.assertBusy(ESTestCase.java:877)
        at org.elasticsearch.xpack.watcher.history.HistoryTemplateTransformMappingsTests.testTransformFields(HistoryTemplateTransformMappingsTests.java:74)

x-pack/plugin/watcher/src/test/java/org/elasticsearch/xpack/watcher/history/HistoryTemplateTransformMappingsTests.java

@@ -81,6 +81,7 @@ public void testTransformFields() throws Exception {
             // time might have rolled over to a new day, thus we need to check that this field exists only in one of the history indices
             List<Boolean> payloadNulls = response.mappings().values().stream()
                     .map(map -> map.get("doc"))
+                    .filter(Objects::nonNull)
                     .map(map -> map.get("result.actions.transform.payload"))
                     .filter(Objects::nonNull)
                     .map(GetFieldMappingsResponse.FieldMappingMetaData::isNull)