[4.0] Fix joomla.edit.params when render subform with fieldset

[Fedik]

Summary of Changes

This fix a bug, when the params tab become empty if one of the field is a subform with own fieldset

Testing Instructions

Apply patch.
Add next subform field to Custom HTML module (in <fieldset name="options"):

<field name="test_fields" type="subform" label="test_fields"
  filter="FooBar::filterTest" multiple="true" min="1" groupByFieldset="true">
   <form>
    <fieldset name="test1" label="fieldset 1">
     <field name="test_field" type="text" label="TESTFIELD1"/>
    </fieldset>
    <fieldset name="test2" label="fieldset 2">
     <field name="test_field2" type="text" label="TESTFIELD2"/>
    </fieldset>
   </form>
</field>

Open the module for editing, check the options tab (or where field was added)

Expected result

you see there all fields, and subform field also

Actual result

the tab are empty

[Fedik]

@N6REJ please test

[N6REJ]

could not test via patchtester so applied changes manually and it works properly.

[Quy]

I have tested this item ✅ successfully on 1be3055

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/27563.}

[Quy]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/27563.}

[wilsonge]

Thanks!