fix: bulk insert empty elements

Bulk insert

package.json

@@ -1,6 +1,6 @@
 {
   "name": "fixed-merkle-tree",
-  "version": "0.6.0",
+  "version": "0.6.1",
   "description": "Fixed depth merkle tree implementation with sequential inserts",
   "repository": "https://github.com/tornadocash/fixed-merkle-tree.git",
   "main": "src/merkleTree.js",

src/merkleTree.js

@@ -78,6 +78,10 @@ class MerkleTree {
    * @param {Array} elements Elements to insert
    */
   bulkInsert(elements) {
+    if (!elements.length) {
+      return
+    }
+
     if (this._layers[0].length + elements.length > this.capacity) {
       throw new Error('Tree is full')
     }