fix: lean-imt-update-member

When a member is removed from the tree (update a leaf with 0n) and then another member is updated, the root that is saved is not correct, so the proof that is generated is not valid.This was caused by a type validation failure.
This fix the behavior described above.
I detected this when I was testing in Semphore groups.

packages/lean-imt/src/lean-imt.ts

@@ -222,7 +222,7 @@ export default class LeanIMT<N = bigint> {
                 // (like the 'insert' function).
                 const sibling = this._nodes[level][index + 1]
 
-                if (sibling) {
+                if (sibling !== undefined) {
                     node = this._hash(node, sibling)
                 }
             }

packages/lean-imt/tests/lean-imt.test.ts

@@ -443,6 +443,30 @@ describe("Lean IMT", () => {
             }
             expect(LeanIMT.verifyProof(proof, badHash)).toBe(false)
         })
+
+        it(`Should insert members,remove member,update member and verifyProof`, () => {
+            const tree = new LeanIMT(poseidon)
+
+            tree.insert(BigInt(1))
+
+            tree.insert(BigInt(2))
+
+            tree.insert(BigInt(3))
+
+            // Remove the third member.
+            tree.update(2, BigInt(0))
+
+            let proof = tree.generateProof(1)
+
+            expect(tree.verifyProof(proof)).toBe(true)
+
+            // Update the second member.
+            tree.update(1, BigInt(2))
+
+            // Validating a proof generated by the first member
+            proof = tree.generateProof(1)
+            expect(tree.verifyProof(proof)).toBe(true)
+        })
     })
 
     describe("# import/export", () => {