Add default value that reduces RTwSA to RT

Author: Gereon A. Kaiping

src/beast/evolution/tree/RandomTreeWithSA.java

@@ -1,18 +1,20 @@
 package beast.evolution.tree;
 
+import java.util.ArrayList;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Set;
 
 import beast.core.Input;
 import beast.core.Input.Validate;
 import beast.core.util.Log;
+import beast.evolution.alignment.Taxon;
 import beast.evolution.alignment.TaxonSet;
 
 public class RandomTreeWithSA extends RandomTree {
 
-	final public Input<TaxonSet> startAsSampledAncestors = new Input<>("sampledAncestor",
-			"Taxa to start as sampled ancestors", Validate.REQUIRED);
+	final public Input<TaxonSet> startAsSampledAncestors = new Input<TaxonSet>("sampledAncestor",
+			"Taxa to start as sampled ancestors", new TaxonSet(new ArrayList<Taxon>()));
 
 	@Override
 	public void initAndValidate() {