fix collection first() and null-key existence handling

[techmahedy]

Description

This PR fixes two issues in Phaseolies\Support\Collection and adds unit coverage to prevent regressions.

What changed

• Fixed Collection::first() so it returns the first actual item in the collection, even when the collection uses associative keys.
• Fixed Collection::__isset() and Collection::offsetExists() to use array_key_exists() instead of isset(), so keys with null values are still treated as present.