Does the fact that the company AIR NOSTRUM compensates a group such as cabin crew, where the majority of the individuals making up the group are women, for the expenses which they have to meet when travelling, other than those related to transport and accommodation, with an amount smaller than that received for the same expenses by another group of employees, such as pilots, in which the majority are men, constitute an instance of indirect discrimination on grounds of sex in relation to working conditions, contrary to European Union law and prohibited under Article 14(1)(c) of Directive 2006/54, where the reason for such different treatment lies in the fact that each group is subject to a different collective agreement, both negotiated by the same company but with different union representatives, pursuant to Article 87 of the Estatuto de los Trabajadores (Spanish Workers’ Statute; ‘the Workers’ Statute’)?