[4.1] Prevent disabling a parent template when children exist

[dgrammatiko]

Pull Request for Issue # .

Summary of Changes

To prevent a possible disaster (disabling a parent template but child still around and default) a simple check was added in the installer component. (FWIW if a parent is disabled the child template is fully functional for the Cassiopeia but if a template also disables some plugins, etc then it might break)

Testing Instructions

• Install https://github.com/joomla/joomla-cms/files/7235154/cassy_1.0.0.zip
• Create a child template
• try to disable Cassy
• uninstall the child then try to disable Cassy

Actual result BEFORE applying this Pull Request

A parent template could be disabled although there might be children

Expected result AFTER applying this Pull Request

Disabling is not possible if children exist

Documentation Changes Required

[khu5h1]

I have tested this item ✅ successfully on f00ce49

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/36192.}

[Quy]

I have tested this item ✅ successfully on 7f08205

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/36192.}

[Quy]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/36192.}

[bembelimen]

Thx