V2023 02 24+php8 corrections

[ternite]

This pull request removes a lot of warning that still arose when I created ODT documents in my specific environment.

As of now, all warnings I saw were resolved. My use case involves a good bunch of plugins which also need attendance when upgrading to PHP 8. Unfortunately, when correcting some of their warnings, new warnings came up in the ODT plugin. Probably because code was traversed within ODT, now, which was omitted previously.

I can't resolve all plugin issues in all plugins I use, right now, so it may be that I will provide more PHP 8 corrections to the ODT plugin when I went over the rest of the plugins (or ideally, when their maintainers provided a PHP 8 fix update).

/resolves #286

[Klap-in]

Looks good so far, shall wait a bit with merging until you finished your round along the other plugins?

[ternite]

Ok. I'll certainly provide fixes when they arise during my code reviews of the other plugins.

I cannot say though, when this will be. It's not the highest priority at the moment, since there is no urgent need to upgrade to PHP 8 in our production environment, yet.

Feel free to merge the pull request, anytime, if you believe it's necessary. I'd create a new separate pull request then.

[Klap-in]

Then I merge it already. Just push more to this branch to continue. Thanks!