Fix TrackMate sometimes crashing with NPE on loading last settings.

The manual tracker factory returns null as for default settings,
which cause issues when trying to use this as default to
deserialize the last used settings.
Noticed by @lxenard

Author: tinevez

src/main/java/fiji/plugin/trackmate/detection/ManualDetectorFactory.java

@@ -65,7 +65,6 @@ public boolean has2Dsegmentation()
 		return true;
 	}
 
-
 	@Override
 	public ConfigurationPanel getDetectorConfigurationPanel( final Settings settings, final Model model )
 	{

src/main/java/fiji/plugin/trackmate/tracking/manual/ManualTrackerFactory.java

@@ -21,6 +21,7 @@
  */
 package fiji.plugin.trackmate.tracking.manual;
 
+import java.util.Collections;
 import java.util.Map;
 
 import javax.swing.ImageIcon;
@@ -92,6 +93,6 @@ public String checkSettings( final Map< String, Object > settings )
 	@Override
 	public Map< String, Object > getDefaultSettings()
 	{
-		return null;
+		return Collections.emptyMap();
 	}
 }