Add missing unit test to reach 100%

Signed-off-by: huy pham <huyp@amazon.com>

Author: huypham612

data-prepper-plugins/otlp-sink/src/test/java/org/opensearch/dataprepper/plugins/sink/otlp/http/OtlpHttpSenderTest.java

@@ -422,4 +422,20 @@ void testHandleSuccessfulResponse_privateMethod_parseException() throws Exceptio
         verify(mockEventHandle2).release(true);
         verify(mockEventHandle3).release(true);
     }
+
+    @Test
+    void testHandleResponse_withNullResponseBody_logsNoBody() throws Exception {
+        final Method method = OtlpHttpSender.class.getDeclaredMethod("handleResponse", int.class, byte[].class, List.class);
+        method.setAccessible(true);
+
+        method.invoke(sender, 500, null, testBatch);
+
+        verify(metrics).recordResponseCode(500);
+        verify(metrics).incrementRejectedSpansCount(SPANS_COUNT);
+
+        // Verifying event handles released with success=false
+        verify(mockEventHandle1).release(false);
+        verify(mockEventHandle2).release(false);
+        verify(mockEventHandle3).release(false);
+    }
 }