Should European Union law and, more specifically, Article 24 of Directive 2006/54/EC of the European Parliament and of the Council of 5 July 2006 on the implementation of the principle of equal opportunities and equal treatment of men and women in matters of employment and occupation, be interpreted as precluding national legislation which affords protection against retaliation to persons who act as witnesses only to persons who, in the context of the investigation of a complaint, bring to the notice of the person with whom the complaint is lodged, in a signed and dated document, the facts which they have personally seen or heard and which relate to the situation which is the subject of the complaint filed or who appear as witnesses in legal proceedings?