Dont put report log into bundle if it's null.

bug:25738644
Change-Id: If0c6c349fd95c54973062776388d5f8a76b3d950
diff --git a/libs/deviceutil/src/android/cts/util/DeviceReportLog.java b/libs/deviceutil/src/android/cts/util/DeviceReportLog.java
index efc31f0..4dd185e 100644
--- a/libs/deviceutil/src/android/cts/util/DeviceReportLog.java
+++ b/libs/deviceutil/src/android/cts/util/DeviceReportLog.java
@@ -45,7 +45,7 @@
     public void deliverReportToHost(Instrumentation instrumentation) {
         Log.i(TAG, "deliverReportToHost");
         String report = generateReport();
-        if (!report.equals("")) {
+        if (report != null && !report.isEmpty()) {
             Bundle output = new Bundle();
             output.putString(CTS_RESULT_KEY, report);
             instrumentation.sendStatus(INST_STATUS_IN_PROGRESS, output);