checks key for null in getData method of MPRISource

fixes issue 4155

Change-Id: I26fcb0b617339e3db81bef0836436c4920d57f4f
Reviewed-on: http://gerrit.dmdirc.com/1263
Reviewed-by: Gregory Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Local Commits <dmdirc@googlemail.com>

src/com/dmdirc/addons/mediasource_dbus/MPRISSource.java

@@ -101,7 +101,8 @@ public class MPRISSource implements MediaSource {
      * @return The value of the specified key or "Unknown".
      */
     protected String getData(final String key) {
-        return data == null || !data.containsKey(key) ? "Unknown" : data.get(key);
+        return data == null || !data.containsKey(key) || data.get(key) == null
+                ? "Unknown" : data.get(key);
     }
 
     /** {@inheritDoc} */