Do Article 168(a) of Council Directive 2006/112/EC and the principle of VAT neutrality preclude a national practice where the right is granted to a full deduction of input tax in connection with the purchase of goods and services used both for the purposes of a taxable person's transactions falling within the scope of VAT (taxed and exempted) and falling outside the scope of VAT, owing to the absence in national law of methods and criteria for apportioning the input tax in relation to those types of transaction?