Add null check in InternalSearchHit#sourceRef to prevent NPE (#21431)

Add null check in InternalSearchHit#sourceRef to prevent NPE

Closes #19279

core/src/main/java/org/elasticsearch/search/internal/InternalSearchHit.java

@@ -200,6 +200,10 @@ public class InternalSearchHit implements SearchHit {
      */
     @Override
     public BytesReference sourceRef() {
+        if (this.source == null) {
+            return null;
+        }
+
         try {
             this.source = CompressorFactory.uncompressIfNeeded(this.source);
             return this.source;

core/src/test/java/org/elasticsearch/search/internal/InternalSearchHitTests.java

@@ -76,4 +76,15 @@ public class InternalSearchHitTests extends ESTestCase {
         assertThat(results.getAt(1).shard(), equalTo(target));
     }
 
+    public void testNullSource() throws Exception {
+        InternalSearchHit searchHit = new InternalSearchHit(0, "_id", new Text("_type"), null);
+
+        assertThat(searchHit.source(), nullValue());
+        assertThat(searchHit.sourceRef(), nullValue());
+        assertThat(searchHit.sourceAsMap(), nullValue());
+        assertThat(searchHit.sourceAsString(), nullValue());
+        assertThat(searchHit.getSource(), nullValue());
+        assertThat(searchHit.getSourceRef(), nullValue());
+        assertThat(searchHit.getSourceAsString(), nullValue());
+    }
 }