fix: validation when key does not exists

[michalsn]

Description
Fixes a bug where wildcard field validation (e.g., contacts.friends.*.name) silently skipped array elements that were missing the validated key whenever at least one sibling element had it present. Only the elements whose key existed in the data were validated. The rest were invisible to all rules.

The root cause is that the validator builds $values by filtering a flattened array - keys absent from the data simply never appear, so there is nothing to validate against. For non-wildcard fields, this is not a problem because
dot_array_search() already returns null for a missing key. Wildcard fields had no equivalent treatment.

Fixes #8006

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide