IMT `new()` does not initialize correctly when leaves are not provided

[0xjei]

Describe the bug
hello, when you try to create a new IMT without providing any leaves, the latest level (root) does not contain the latest zero_value, as happens for the TS implementation.

In fact, there's no test covering the IMT new() with an empty leaves vector.

To Reproduce
Steps to reproduce the behavior:

1. call new() with an empty leaves vector.

Expected behavior
I'm expecting the root to have the latest zero_value.

Rust

TS