BaseCollection::first now returns an Option

Summary:
Previous behavior: return first item or throw an option on empty collection
Current behavior: similar to Rust, return Some(first item) or None

The behavior of BaseCollection::firstOr is not changed, with eager loading.
For lazy loading, we can now use $collection->first()->getValueOrElse().

Test Plan:

• Unit tests have been updated
• No-op for Zed code (it uses firstOr)

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3852