fix: called option unwrap on a none value on insert

[edwinkys]

Purpose

This PR fixes a bug that happens when attempting to insert a new vector record to the collection but the collection can't find the first valid vector ID to start the insert operation.

Testing

• [x] I have tested this PR locally.
• [x] I added tests to cover my changes, if not applicable, I have added a reason why.

The script below will reproduce the issue with the previous version:

use oasysdb::prelude::*;

fn main() {
    let config = Config::default();
    let mut collection = Collection::new(&config);
    let record = Record::many_random(1536, 2);

    // Insert and delete a record
    collection.insert(&record[0]).unwrap();
    collection.delete(&VectorID(0)).unwrap();

    // Insert another record
    collection.insert(&record[1]).unwrap();
}

Chore checklist

• [x] I have updated the documentation accordingly.
• [x] I have added comments to most of my code, particularly in hard-to-understand areas.