Fall back to index.php if file not found.

[Bakual]

Pull Request for Issue #10207 .

Summary of Changes

This PR simply adds a fallback to index.php in case the requested file isn't present neither in the active template nor in the system template

Testing Instructions

Test by adding ?tmpl= or ?tmpl=foo to the URL. Without this PR you will get a blank page, with this PR the page will be rendered with the system/index,php file (looks ugly but it renders).

[andrepereiradasilva]

I have tested this item ✅ successfully on 739d22b

looks ugly but it renders

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[infograf768]

I have tested this item ✅ successfully on 739d22b

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[infograf768]

RTC.Thanks.

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