fix: Markle tree: do not try to read rows if no one exists

duniter4j-core-shared/src/main/java/org/duniter/core/util/merkle/MerkleTree.java

@@ -142,6 +142,7 @@ public class MerkleTree {
     }
 
     public String root() {
+        if (nodesCount == 0) return null; // No leaves
         return rows.get(0).get(0);
     }