Avoid NPE in `more_like_this` when field has zero tokens (#30365)

Fixes and edge case when using `more_like_this` where TermVectorsWriter
could throw an NPE when a field produced zero tokens after analysis. This
changes the implementation to use an empty list of tokens in this case.

Closes #30148

docs/CHANGELOG.asciidoc

@@ -104,6 +104,8 @@ ones that the user is authorized to access in case field level security is enabl
 [float]
 === Bug Fixes
 
+Fix NPE in 'more_like_this' when field has zero tokens ({pull}30365[#30365])
+
 Fixed prerelease version of elasticsearch in the `deb` package to sort before GA versions
 ({pull}29000[#29000])
 
@@ -169,6 +171,8 @@ Added put index template API to the high level rest client ({pull}30400[#30400])
 [float]
 === Bug Fixes
 
+Fix NPE in 'more_like_this' when field has zero tokens ({pull}30365[#30365])
+
 Do not ignore request analysis/similarity settings on index resize operations when the source index already contains such settings ({pull}30216[#30216])
 
 Fix NPE when CumulativeSum agg encounters null value/empty bucket ({pull}29641[#29641])

server/src/main/java/org/elasticsearch/action/termvectors/TermVectorsWriter.java

@@ -70,6 +70,10 @@ final class TermVectorsWriter {
             Terms topLevelTerms = topLevelFields.terms(field);
 
             // if no terms found, take the retrieved term vector fields for stats
+            if (fieldTermVector == null) {
+                fieldTermVector = EMPTY_TERMS;
+            }
+
             if (topLevelTerms == null) {
                 topLevelTerms = EMPTY_TERMS;
             }

server/src/test/java/org/elasticsearch/search/morelikethis/MoreLikeThisIT.java

@@ -91,6 +91,36 @@ public class MoreLikeThisIT extends ESIntegTestCase {
         assertHitCount(response, 1L);
     }
 
+    //Issue #30148
+    public void testMoreLikeThisForZeroTokensInOneOfTheAnalyzedFields() throws Exception {
+        CreateIndexRequestBuilder createIndexRequestBuilder = prepareCreate("test")
+            .addMapping("type", jsonBuilder()
+            .startObject().startObject("type")
+                .startObject("properties")
+                .startObject("myField").field("type", "text").endObject()
+                .startObject("empty").field("type", "text").endObject()
+                .endObject()
+            .endObject().endObject());
+
+        assertAcked(createIndexRequestBuilder);
+
+        ensureGreen();
+
+        client().index(indexRequest("test").type("type").id("1").source(jsonBuilder().startObject()
+            .field("myField", "and_foo").field("empty", "").endObject())).actionGet();
+        client().index(indexRequest("test").type("type").id("2").source(jsonBuilder().startObject()
+            .field("myField", "and_foo").field("empty", "").endObject())).actionGet();
+
+        client().admin().indices().refresh(refreshRequest()).actionGet();
+
+        SearchResponse searchResponse = client().prepareSearch().setQuery(
+            moreLikeThisQuery(new String[]{"myField", "empty"}, null, new Item[]{new Item("test", "type", "1")})
+                .minTermFreq(1).minDocFreq(1)
+        ).get();
+
+        assertHitCount(searchResponse, 1L);
+    }
+
     public void testSimpleMoreLikeOnLongField() throws Exception {
         logger.info("Creating index test");
         assertAcked(prepareCreate("test")