[BUGFIX] Added extra checks within the metatag generators in case fields do not exist, fixed hasSitemapFields method with correct property

[RinyVT]

Summary

This PR can be summarized in the following changelog entry:

• Metatag generators could throw errors trying to access fields which were not available
• hasSitemapFields incorrectly returned yoastSeoFields instead of sitemapFields

Relevant technical choices:

• Added ?? checks to the possibly non-existing fields

Test instructions

This PR can be tested by following these steps:

• Pull branch
• Test if setSitemapFields(false) really does not add the fields in the backend
• Test if the frontend does not throw errors when fields in the database (like og_image) do no exist

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

Fixes #550