[TEST] add test for case where search source is null (#28398)

Closes #28393

Author: javanna

client/rest-high-level/src/test/java/org/elasticsearch/client/RequestTests.java

@@ -30,10 +30,10 @@
 import org.apache.http.entity.StringEntity;
 import org.apache.http.util.EntityUtils;
 import org.elasticsearch.action.DocWriteRequest;
-import org.elasticsearch.action.admin.indices.alias.get.GetAliasesRequest;
-import org.elasticsearch.action.admin.indices.close.CloseIndexRequest;
 import org.elasticsearch.action.admin.indices.alias.IndicesAliasesRequest;
 import org.elasticsearch.action.admin.indices.alias.IndicesAliasesRequest.AliasActions;
+import org.elasticsearch.action.admin.indices.alias.get.GetAliasesRequest;
+import org.elasticsearch.action.admin.indices.close.CloseIndexRequest;
 import org.elasticsearch.action.admin.indices.create.CreateIndexRequest;
 import org.elasticsearch.action.admin.indices.delete.DeleteIndexRequest;
 import org.elasticsearch.action.admin.indices.mapping.put.PutMappingRequest;
@@ -800,6 +800,14 @@ public void testBulkWithDifferentContentTypes() throws IOException {
         }
     }
 
+    public void testSearchNullSource() throws IOException {
+        SearchRequest searchRequest = new SearchRequest();
+        Request request = Request.search(searchRequest);
+        assertEquals(HttpPost.METHOD_NAME, request.getMethod());
+        assertEquals("/_search", request.getEndpoint());
+        assertNull(request.getEntity());
+    }
+
     public void testSearch() throws Exception {
         String[] indices = randomIndicesNames(0, 5);
         SearchRequest searchRequest = new SearchRequest(indices);