[BUGFIX] Show warning on linking suggestions when content element is set to 'All Languages' instead of fatal exception

[RinyVT]

Summary

This PR can be summarized in the following changelog entry:

• Internal linking suggestions should show a warning within a content element with Language "All" (-1) instead of throwing an exception

Relevant technical choices:

• Moved the getLanguageById(.... call into the try/catch block to catch the InvalidArgumentException, if the $locale is null the warning is rendered within the template

Test instructions

This PR can be tested by following these steps:

• Pull branch
• Add a content element with Language "All"
• Check if the warning is shown within the Internal Linking Suggestions

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 #543