fix(select-tree): handle empty strict root results

[CodeWithDennis]

Summary

Fixes a production crash in the select tree view when strict null parent root nodes are enabled and the result set does not contain an entry for the configured root parent value.

In that situation, buildTreeFromResults() tried to read $resultCache[$parent]['children'] directly. When the category tree is empty, or when filtering leaves no valid root bucket, $resultCache[$parent] is not initialized. PHP then raises an Undefined array key "" warning, which Laravel converts into a ViewException while rendering resources/views/select-tree.blade.php.

Changes

• Treat missing strict-root children as an empty array instead of reading the array key unconditionally.
• Preserve the existing behavior for non-strict mode, where orphaned children can still be promoted to root nodes.

Why

The component should be able to render an empty tree without failing. An empty category, an empty query result, or a filtered tree with no root nodes are valid states for the field and should produce an empty option list.

Verification

• Ran php -l src/SelectTree.php

The full Pest/PHPUnit suite was not run locally.