Make TreeUtils.getMinTimepoint() return 0 in case of an empty model.

* It was before Integer.MAX_VALUE, which was confusing to be a min time point
* Update javadocs and unit test accordingly

src/main/java/org/mastodon/util/TreeUtils.java

@@ -232,12 +232,16 @@ private static < V extends Vertex<E>, E extends Edge< V > > RefSet< V > filterRo
 	}
 
 	/**
-	 * Gets the minimum timepoint in the given {@link Model} at which at least one {@link Spot} exists in the Model.
+	 * Gets the minimum time point in the given {@link Model} at which at least one {@link Spot} exists in the Model.
+	 * <br>
+	 * If the model is empty, returns 0.
 	 * @param model the {@link Model}
 	 * @return the timepoint
 	 */
 	public static int getMinTimepoint( final Model model )
 	{
+		if ( model.getGraph().vertices().isEmpty() )
+			return 0;
 		int minTimepoint = Integer.MAX_VALUE;
 		for ( final Spot spot : model.getGraph().vertices() )
 			minTimepoint = Math.min( minTimepoint, spot.getTimepoint() );

src/test/java/org/mastodon/util/TreeUtilsTest.java

@@ -41,6 +41,7 @@
 import org.mastodon.collection.RefSet;
 import org.mastodon.mamut.feature.branch.exampleGraph.ExampleGraph1;
 import org.mastodon.mamut.feature.branch.exampleGraph.ExampleGraph2;
+import org.mastodon.mamut.model.Model;
 import org.mastodon.mamut.model.ModelGraph;
 import org.mastodon.mamut.model.Spot;
 
@@ -182,7 +183,7 @@ public void testGetMinTimepoint()
 	{
 		assertEquals( 0, TreeUtils.getMinTimepoint( new ExampleGraph1().getModel() ) );
 		assertEquals( 0, TreeUtils.getMinTimepoint( new ExampleGraph2().getModel() ) );
-
+		assertEquals( 0, TreeUtils.getMinTimepoint( new Model() ) );
 	}
 
 	@Test