[BUGFIX] Fixed loading state of internal linking suggestions when bodytext is empty (e.g. new content element)

[RinyVT]

Summary

This PR can be summarized in the following changelog entry:

• When the bodytext was empty, the internal linking suggestions would be kept in a loading state because the loading state is triggered with an empty "content" variable. This PR fixes that.

Relevant technical choices:

• Instead of checking if the content is empty, the getCKEditorContent now returns null if the CKEditor has not yet finished loading. This way, it's still possible to have empty content but the loading state is not dependent on it anymore.

Test instructions

This PR can be tested by following these steps:

• Pull branch and check if the loading state now disappears when CKeditor is done loading

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 #536