Largest product of two neighbors

Write a function maxAdjacentProduct(numbers) that returns the largest product of two neighbor elements of an array.

• numbers is an array of integers. They can be positive, negative or 0.
• Two elements are neighbors when their indexes are i and i + 1. Multiply every such pair and return the largest product as a number.
• Two negative numbers give a positive product: -5 * -6 is 30.
• All products can be negative. Then return the largest of them, for example -2.
• If the array has fewer than two elements, there are no pairs: return null.