Bugfix SVG logo upload

[lenaherf]

Note: Please fill out all relevant sections and remove irrelevant ones.

🔀 Purpose of this PR:

• Fixes a bug
• Updates for a new Moodle version
• Adds a new feature of functionality
• Improves or enhances existing features
• Refactoring: restructures code for better performance or maintainability
• Testing: add missing or improve existing tests
• Miscellaneous: code cleaning (without functional changes), documentation, configuration, ...

---

📝 Description:

This PR fixes the bug that appeared when uploading an SVG file with a format that was not supported. This threw an uncaught DivisionByZero error on PHP 8+.

The root cause is in classes/output_image.php, method get_logo_aspect_ratio(): the function unconditionally reads index [2] and [3] from the exploded viewBox attribute. If the uploaded SVG has no viewBox attribute, these indices are undefined and the resulting '' / '' division crashes the request on PHP 8+.

With the bugfix it is possible to also upload SVG files that only have width/heigt attributes.

---

📋 Checklist

Please confirm the following (check all that apply):

• I have phpunit and/or behat tests that cover my changes or additions.
• Code passes the code checker without errors and warnings.
• Code passes the moodle-ci/cd pipeline on all supported Moodle versions or the ones the plugin supports.
• Code does not have var_dump() or var_export or any other debugging statements (or commented out code) that
  should not appear on the productive branch.
• Code only uses language strings instead of hard-coded strings.
• If there are changes in the database: I updated/created the necessary upgrade steps in db/upgrade.php and
  updated the version.php.
• If there are changes in javascript: I build new .min files with the grunt amd command.
• If it is a Moodle update PR: I read the release notes, updated the version.php and the CHANGES.md.
  I ran all tests thoroughly checking for errors. I checked if bootstrap had any changes/deprecations that require
  changes in the plugins UI.

---

🔍 Related Issues

• Related to #64

---

🧾📸🌐 Additional Information (like screenshots, documentation, links, etc.)

Any other relevant information.

---