[#7173] Prevent error on disabled performance API

[Luzifer]

What it does

When performance API is disabled through privacy.resistFingerprinting setting in recent Firefox all API calls are possible though getEntriesByName will return an empty Array.

This change checks the length of the returned array before using the first element in order not to cause a call to undefined.duration which breaks plugin architecture.

closes #7173

How to test

In short: Enable resistFingerprinting in Firefox, ensure plugins are still loaded.

Please see description of #7173 for further information.

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[akosyakov]

looks good, please squash commits which fixing issues in the original commit, see https://github.com/eclipse-theia/theia/blob/master/doc/pull-requests.md#checklist-commit-history