Bug #5329 - adding a treatment to purge empty WYSIWYG contents

(adding JCR unit tests ...)
(adding a treatment in order to rename physical WYSIWYG file name according to the language)

The commit of this PR must to be applied on 5.14.x and master version too.

Please don't forget to check the PR https://github.com/Silverpeas/Silverpeas-Core/pull/513.