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

[medmahmoudhdaya]

This PR fixes an issue where Arr::dot() returned empty arrays instead of null when flattening nested structures.

Before:

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

After

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

this makes Arr::dot() more consistent for serialization and database use cases.

Fixes #57305