add unit test with null input

Signed-off-by: yuye-aws <[email protected]>

src/test/java/org/opensearch/neuralsearch/processor/TextChunkingProcessorTests.java

@@ -610,6 +610,17 @@ public void testExecute_withFixedTokenLength_andSourceDataListWithHybridType_the
         assertEquals(((List<?>) listResult).size(), 0);
     }
 
+    @SneakyThrows
+    public void testExecute_withFixedTokenLength_andSourceDataNull_thenSucceed() {
+        TextChunkingProcessor processor = createFixedTokenLengthInstance(createStringFieldMap());
+        IngestDocument ingestDocument = createIngestDocumentWithSourceData(null);
+        IngestDocument document = processor.execute(ingestDocument);
+        assert document.getSourceAndMetadata().containsKey(INPUT_FIELD);
+        Object listResult = document.getSourceAndMetadata().get(OUTPUT_FIELD);
+        assert (listResult instanceof List);
+        assertEquals(((List<?>) listResult).size(), 0);
+    }
+
     @SneakyThrows
     public void testExecute_withDelimiter_andSourceDataString_thenSucceed() {
         TextChunkingProcessor processor = createDelimiterInstance();