[12.26] Fix Arr::dot() to convert empty arrays to null

[medmahmoudhdaya]

This PR updates the Arr::dot() method to convert empty arrays to null when flattening multi-dimensional arrays. The tests have been updated to reflect this behavior.

Before

Arr::dot(['foo' => []]);
// ['foo' => []]

After

Arr::dot(['foo' => []]);
// ['foo' => null]

Converting empty arrays to null makes the output more consistent for serialization, database storage, and logging.

Fixes #57305

[cosmastech]

See Taylor's comment here: #57307 (comment)