[4.0] [plg_system_cache] Fix debug with page caching enabled

[SharkyKZ]

Summary of Changes

Loads a document into application for plugins that expect it to be available in onAfterRespond event.

Testing Instructions

Enable Debug System option in Global Configuration.
Enable System - Page Cache plugin.
Enable System - Debug plugin.
As a guest view the same page in frontend at least twice.

Expected result

No errors.

Actual result

Call to a member function getType() on null

Documentation Changes Required

Page caching and onPageCacheSetCaching event moved from onAfterInitialise to onAfterRoute.

[jwaisner]

I have tested this item ✅ successfully on 2e3ab6f

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/27166.}

[jwaisner]

I have tested this item ✅ successfully on 9b215ba

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/27166.}

[Quy]

I have tested this item ✅ successfully on 9b215ba

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/27166.}

[Quy]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/27166.}