test: fix assertion for okhttp3 5 non-null body

Author: ricellis

modules/common/src/test/java/com/ibm/cloud/cloudant/internal/ErrorTransformInterceptorTest.java

@@ -340,7 +340,7 @@ void runTest(Interceptor interceptor) throws IOException {
         } else {
           // If we aren't asserting a specific part of augment check the body is unchanged
           if ("HEAD" == r.request().method()) {
-            assertNull(r.body());
+            assertEquals(r.body(), ResponseBody.EMPTY);
           } else {
             assertEquals(r.body().string(), responseBody); 
           }