Try This

@api.model
def _default_products(self):
    ids = self.env['stock.move.line'].search([]).mapped('product_id').ids
    domain = [ ('id', 'in', ids) ]
    return self.env['product.product'].search(domain, limit=1)

products_pack = fields.Many2one('product.product',string="Product Name",default=_default_products)