fix: VariationTree not always validating the root

The assertion that the given root node in the constructors of
VariationTree was not always checked, in particular not for the default
constructor by the record.

Author: pmbittner

src/main/java/org/variantsync/diffdetective/variation/diff/construction/GumTreeDiff.java

@@ -48,7 +48,7 @@ public static <L extends Label> VariationDiff<L> diffUsingMatching(VariationTree
             matcher
         );
 
-        return new VariationDiff<>(root, new CompositeSource("diffUsingMatching", before.source(), after.source()));
+        return new VariationDiff<>(root, new CompositeSource("diffUsingMatching", before, after));
     }
 
     /**

src/main/java/org/variantsync/diffdetective/variation/tree/VariationTree.java

@@ -34,18 +34,16 @@
 /**
  * Representation of a concrete variation tree with source information.
  *
- * @param root the root of the variation tree
- * @param source from which source code the variation tree was obtained
  * @param <L> The type of label stored in this tree.
  *
  * @see VariationTreeNode
  * @author Benjamin Moosherr
  */
-public record VariationTree<L extends Label>(
-    VariationTreeNode<L> root,
-    Source source
-) implements Source {
-    /** Creates a {@code VariationTree} with the given root and an unknown source. */
+public class VariationTree<L extends Label> implements Source {
+    private final VariationTreeNode<L> root;
+    private final Source source;
+
+    /** Creates a {@code VariationTree} with the given root and an {@link Source::Unknown unknown} source. */
     public VariationTree(VariationTreeNode<L> root) {
         this(root, Source.Unknown);
     }
@@ -143,7 +141,7 @@ public static <T extends VariationNode<T, L>, L extends Label> VariationTree<L>
     public VariationDiff<L> toVariationDiff(final Function<VariationTreeNode<L>, DiffNode<L>> nodeConverter) {
         return new VariationDiff<>(
                 DiffNode.unchanged(nodeConverter, root()),
-                new CompositeSource("VariationTree.toVariationDiff", source())
+                new CompositeSource("VariationTree.toVariationDiff", source)
         );
     }
 
@@ -235,8 +233,16 @@ public void assertConsistency() {
         forAllPreorder(VariationTreeNode::assertConsistency);
     }
 
+    public VariationTreeNode<L> root() {
+        return root;
+    }
+
+    /**
+     * Returns the source of this VariationTree (i.e., the data this VariationTree was created from).
+     * @see Source
+     */
     public Source getSource() {
-        return source();
+        return source;
     }
 
     @Override