fix inverted logic to append reference URL form field

Bug: 27497448
Change-Id: I196c2945f5f27c07167ce5fb9bbb400611a933a0
diff --git a/common/util/src/com/android/compatibility/common/util/ResultUploader.java b/common/util/src/com/android/compatibility/common/util/ResultUploader.java
index 67b359e..7348284 100644
--- a/common/util/src/com/android/compatibility/common/util/ResultUploader.java
+++ b/common/util/src/com/android/compatibility/common/util/ResultUploader.java
@@ -49,7 +49,7 @@
         try {
             byte[] data = getBytes(input);
             mMultipartForm.addFormFile("resultXml", "test-result.xml.gz", data);
-            if (referenceUrl == null || referenceUrl.trim().isEmpty()) {
+            if (referenceUrl != null && !referenceUrl.trim().isEmpty()) {
                 mMultipartForm.addFormValue("referenceUrl", referenceUrl);
             }
             return mMultipartForm.submit();