Fix NPE in JobStore when jobs.xml is empty.

It is possible that jobs.xml does not have valid content
if the device meet unexpected power off or reboot during
writing the file, then parser.getName() may get null string
on next boot.

Because log does not allow to print null message.
It will result boot fail:
Caused by: java.lang.NullPointerException: println needs a message
 at android.util.Log.println_native(Native Method)
 at android.util.Slog.d
 at com.android.server.job.JobStore
    $ReadJobMapFromDiskRunnable.readJobMapImpl

Change-Id: Icd485096e2f00f4028428a9ad95cd0ef66b2dca0

services/core/java/com/android/server/job/JobStore.java

@@ -457,7 +457,7 @@ public class JobStore {
             while (eventType != XmlPullParser.START_TAG &&
                     eventType != XmlPullParser.END_DOCUMENT) {
                 eventType = parser.next();
-                Slog.d(TAG, parser.getName());
+                Slog.d(TAG, "Start tag: " + parser.getName());
             }
             if (eventType == XmlPullParser.END_DOCUMENT) {
                 if (DEBUG) {