Pull Request resolves #47944.

• I read the Generative AI policy and my contribution is either not created with the help of AI or is compatible with the policy and GNU/GPL 2 or later.

Summary of Changes

When a scheduled task crashes on first run, DB row has locked IS NOT NULL but last_execution = NULL. The Scheduled Tasks admin list crashes with:

DateTime::__construct(): Passing null to parameter #1 ($datetime) of type string is deprecated

Root cause: last_execution passed directly to HTMLHelper::_('date', ...) without null guard inside the $item->locked block. Lines 251 and 256 in the same file already use the correct ternary guard pattern — this block was missing it.

Fix: extract formatted date into $lockedSince with the same ternary guard used on line 251.

Testing Instructions

1. Create a scheduled task
2. Set last_execution = NULL and locked = NOW() in #__scheduler_tasks for that task
3. Navigate to System → Scheduled Tasks
4. Before: page crashes with deprecation fatal
5. After: page renders; locked task shows - in tooltip

Actual result BEFORE applying this PR

Fatal deprecation crash — Scheduled Tasks page inaccessible

Expected result AFTER applying this PR

Page renders normally; locked tasks with no prior execution show - in the running-since tooltip

• No documentation changes for guide.joomla.org needed
• No documentation changes for manual.joomla.org needed