[ActivityManager] Fix NullPointerException when dumping ContentProviderRecord.

Sympton: If there is external access without handles, dumpsys will met exception as:
  * ContentProviderRecord{45ed0fc8 u0 com.android.providers.media/.MediaProvider}
...
    externals=
Exception occurred while dumping:
java.lang.NullPointerException
	at com.android.server.am.ContentProviderRecord.dump(ContentProviderRecord.java:171)

Root Cause: External handles may be empty but there is "no handle" count.

Solution: Check null and print with handle, no handle count.

Change-Id: I0ee497096113e5685d491a0d4346f57d398495e6