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Find the first non-consecutive number
Your task is to find the first element of an array that is not consecutive.
E.g. If we have an array [1,2,3,4,6,7,8] then 1 then 2 then 3 then
4 are all consecutive but 6 is not, so that’s the first non-consecutive number.
If the whole array is consecutive then return null or Nothing.
The array will always have at least 2 elements and all elements will be numbers.
The numbers will also all be unique and in ascending order. The numbers could be
positive or negative and the first non-consecutive could be either too!