[13.x] fix bug: enhance handling of empty values in grouping

[comes]

Description

This PR fixes a bug within groupBy in collections. I understand it can be considered as a breaking change in 12.x, so maybe worth to consider to fix this issue in 13.x

How to reproduce

Prepare an array with some values, for demoing reasons, it's easier to spot the bug with preserveKeys: true enabled.

collect([
  10 => [],
  20 => ['user' => 2, 'roles' => []], // missing
  30 => ['user' => 3, 'roles' => [null]],
  40 => ['user' => 4, 'roles' => ['Role_2']]
])->groupBy('roles', preserveKeys: true)
  ->dump();

The output after grouping missing key 20, because of roles is an empty array, while key 30 is an array with a single value of null.

Solution

Inside groupBy function, treat any empty array returned as a grouping key to null before processing. This ensures that items with an empty array are grouped under the “null” bucket rather than being skipped.