Add an EmptyAdapter

[stof]

Unlike the NullAdapter, this adapter is always empty instead of allowing it to paginate an array of null values. Thanks to that, its generic type can fullfil types like AdapterInterface<Foo> with a non-nullable Foo type.

See https://phpstan.org/r/ada3d136-11fa-43f2-a5dc-dd282f008b3e for the difference it makes for phpstan and https://psalm.dev/r/cb09185906 for Psalm.

This EmptyAdapter is mostly meant to be used for conditional returns, where another branch of the execution would return a different adapter implementation (for instance to skip executing a DB query entirely when you know that the parameters you deal with cannot produce any results because they would execute a query with WHERE true = false).