fix text structure NPE when fields have null value

Author: weizijun

x-pack/plugin/text-structure/src/main/java/org/elasticsearch/xpack/textstructure/structurefinder/TextStructureUtils.java

@@ -425,7 +425,10 @@ static Tuple<Map<String, String>, FieldStats> guessMappingAndCalculateFieldStats
             );
         }
 
-        Collection<String> fieldValuesAsStrings = fieldValues.stream().map(Object::toString).collect(Collectors.toList());
+        Collection<String> fieldValuesAsStrings = fieldValues.stream()
+            .filter(value -> value != null)
+            .map(Object::toString)
+            .collect(Collectors.toList());
         Map<String, String> mapping = guessScalarMapping(
             explanation,
             fieldName,

x-pack/plugin/text-structure/src/test/java/org/elasticsearch/xpack/textstructure/structurefinder/TextStructureUtilsTests.java

@@ -1106,6 +1106,16 @@ public void testGuessGeoShape() {
         }
     }
 
+    public void testGuessMappingWithNullValue() {
+        Map<String, String> expected = Collections.singletonMap(TextStructureUtils.MAPPING_TYPE_SETTING, "keyword");
+
+        Consumer<Boolean> testGuessMappingGivenEcsCompatibility = (ecsCompatibility) -> {
+            assertEquals(expected, guessMapping(explanation, "foo", Arrays.asList("ERROR", null, "DEBUG"), ecsCompatibility));
+        };
+
+        ecsCompatibilityModes.forEach(testGuessMappingGivenEcsCompatibility);
+    }
+
     private Map<String, String> guessMapping(
         List<String> explanation,
         String fieldName,