PHP 8.4 implicitly nullable type deprecation notices

[Bilge]

Further to #99, there are now a gorillion PHP 8.4 deprecation notices regarding implicit null types.

Deprecated: Ds\Traits\GenericSequence::filter(): Implicitly marking parameter $callback as nullable is deprecated, the explicit nullable type must be used instead
Deprecated: Ds\Traits\GenericSequence::join(): Implicitly marking parameter $glue as nullable is deprecated, the explicit nullable type must be used instead
Deprecated: Ds\Traits\GenericSequence::slice(): Implicitly marking parameter $length as nullable is deprecated, the explicit nullable type must be used instead
Deprecated: Ds\Traits\GenericSequence::sort(): Implicitly marking parameter $comparator as nullable is deprecated, the explicit nullable type must be used instead
Deprecated: Ds\Traits\GenericSequence::sorted(): Implicitly marking parameter $comparator as nullable is deprecated, the explicit nullable type must be used instead
Deprecated: Ds\Sequence::filter(): Implicitly marking parameter $callback as nullable is deprecated, the explicit nullable type must be used instead
Deprecated: Ds\Sequence::join(): Implicitly marking parameter $glue as nullable is deprecated, the explicit nullable type must be used instead
Deprecated: Ds\Sequence::slice(): Implicitly marking parameter $length as nullable is deprecated, the explicit nullable type must be used instead
Deprecated: Ds\Sequence::sort(): Implicitly marking parameter $comparator as nullable is deprecated, the explicit nullable type must be used instead
Deprecated: Ds\Sequence::sorted(): Implicitly marking parameter $comparator as nullable is deprecated, the explicit nullable type must be used instead

...to name a few.