fix replaceTags function for array

[keewhip]

Pull Request for Issue #38279.

Summary of Changes

If value to replace is an array, it is now replaced with the value
Like user custom fields

Testing Instructions

Create a user required custom field named testfield
Show this field on registration page
Include this field in the new user mailtemplate using the code {COM_FIELDS}testfield{/COM_FIELDS}
Register an new user account

Actual result BEFORE applying this Pull Request

The placeholder is replaced with a empty string

Expected result AFTER applying this Pull Request

The placeholder is replaced with the value submitted by the user

Documentation Changes Required

None

[MacJoom]

I have tested this item ✅ successfully on a209ffb

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/38305.}

[viocassel]

I have tested this item ✅ successfully on a209ffb

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/38305.}

[richard67]

RTC

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/38305.}

[fancyFranci]

Thank you for fixing this bug!

[ReLater]

This change has opened another issue. See #39774