[BUGFIX] DPL-151: Mitigate "Undefined array key" in "LanguageService"

[sbuerk]

Following issue has been reported by a user of the extension:

Core: Exception handler (WEB): Uncaught TYPO3 Exception:
  WebVision\DeeplWrite\Domain\Enum\RephraseSupportedDeepLLanguage::tryFrom():
  Argument #1 ($language) must be of type string, null given, called in
  vendor/web-vision/deepl-write/Classes/Service/LanguageService.php
  on line 18 | TypeError thrown in file
  vendor/web-vision/deepl-write/Classes/Domain/Enum/RephraseSupportedDeepLLanguage.php
  in line 59.

Core: Error handler (BE): PHP Warning: Undefined array key "deeplWriteLanguage"
  in vendor/web-vision/deepl-write/Classes/Service/LanguageService.php
  line 16

In case the language configuration in the SiteConfig has not been
done yet the array key does not exists and the current code tries
to access the array with a key not existing in the array, honered
with the above error and exception.

The array access is now guarded using null-coalsce fallback in
the LangaugeService and also ensuring that the result is really
a string, falling back to an empty string and mitigates the issue.

Functional tests have been added to cover the LanguageService
and one test specifically added for the above mentioned issue
failes if the change to fix the issue is reverted.

Important

Functional test infrastructure for this package not in place
yet and not active in github action workflow. Will be done
in a dedicated change. However, added tests works using the
functional test of the private deepltranslate root development
setup (internally).