[subform] Validation of multiple subform. Fix Invalid argument supplied

[ReLater]

Pull Request for Issue #25308

Summary of Changes

• Add check for empty $value when a multiple subform without rows is sent through validation.

Testing Instructions

• See issue [subform] Invalid argument supplied for foreach() #25308

Expected result

• No PHP warning

Actual result

PHP Warning: Invalid argument supplied for foreach() in \libraries\src\Form\Form.php on line 1558

[Quy]

I have tested this item ✅ successfully on ae39f01

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/25310.}

[richard67]

I have tested this item ✅ successfully on ae39f01

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/25310.}

[ghost]

Status "Ready To Commit".

[HLeithner]

thx