Add Collection::isEmpty Method

[AlexandruGG]

Overview

This PR adds the isEmpty method and IsEmptyable interface.

Why is this needed when we have Collection::count?

1. Convenience

Typical use case:

$collection = Collection::fromIterable([1, 2, 3]);
$filteredCollection = $collection->filter(...);

if (!$filteredCollection->isEmpty()) {
    // do something
}

// compared to

if (0 !== $filteredCollection->count()) {
    // do something
}

Other popular PHP collection libraries also have this, so people are used to it:
Doctrine: https://www.doctrine-project.org/projects/doctrine-collections/en/1.6/index.html#isempty
Laravel: https://laravel.com/docs/8.x/collections#method-isempty
Knapsack: https://dusankasan.github.io/Knapsack/#Operations-isEmpty

Other languages have this:
Java: https://docs.oracle.com/javase/8/docs/api/java/util/Collection.html#isEmpty--
C++: https://www.cplusplus.com/reference/vector/vector/empty/
Haskell: http://zvon.org/other/haskell/Outputprelude/null_f.html

2. Performance

This is a big one. Simple benchmark using microtime and Collection::range(0, 100_000_000):

$isEmpty = 0 === $collection->count();
// vs
$isEmpty = $collection->isEmpty();

Ran it 100,000 times for isEmpty(), only 10 times for count, and here's why (average time for a single run):

isEmpty -> 5.575289726257324E-6 seconds (basically instant because it's in constant time)
count -> 13.571631002426148 seconds (this runs in O(N))

Checklist

• Tests
• Static analysis check
• Documentation

[AlexandruGG]

@drupol I hope you'll like this one 😁

[drupol]

I'm ok for it just a minor thing to do and we are good.

[drupol]

Thanks, good addition !