Fix: null dereference in RestrictionUri::getAssetUrlInformation() when asset has no restriction

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Bug

In RestrictionUri::getAssetUrlInformation():

try {
    $restriction = Restriction::getByTargetId($assetId, 'asset');
} catch (\Exception $e) {
    return null;
}

$userGroups = $restriction->getRelatedGroups();

Restriction::getByTargetId() returns null (not a thrown exception) when there is no restriction row for the asset — the common case of an asset in a protected folder without an explicit restriction record. $restriction->getRelatedGroups() then throws \Error, which is not caught by the catch (\Exception) above (Error is not an Exception), so it surfaces as an uncaught fatal.

Fix

Guard the null case (no restriction → no related groups), consistent with the "restricted mode without any restriction settings → empty groups" handling immediately below.

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Withdrawing — on master (v5) this is not a defect. Restriction\Dao::getByField() throws a \Exception when no row exists, so Restriction::getByTargetId(): self never returns null and the not-found case is already handled by the surrounding catch (\Exception) { return null; }. $restriction is therefore always a Restriction here, making the added instanceof redundant (correctly flagged by phpstan). The nullable behaviour only exists in a downstream fork where getByField() returns null instead of throwing. Apologies for the noise.