Avoid PHP undefined index notices on extension pages

[braders]

Overview

On the extensions page (/civicrm/admin/extensions) various notices (undefined index) are thrown if the extension info.xml files are missing one or more keys. This adds a helper method to set relevant defaults for use in the twig files.

This also removes use of empty() in the templates/CRM/Admin/Page/ExtensionDetails.tpl template which is problematic with the new CIVICRM_SMARTY_DEFAULT_ESCAPE.

Before

Many PHP notices occur on the extension manager page:

After

The bulk of these notices are removed (a Undefined index: tabHeader still occurs, but this is a problem common to many other pages).

[civibot]

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[eileenmcnaughton]

thanks @braders