Create a function that given an array of dates returns the average date in string form.
averageDate([]) β null
averageDate([
new Date(Date.UTC(1970, 0, 1))
]) β "1970-01-01T00:00:00.000Z"
averageDate([
new Date(Date.UTC(1970, 0, 1)),
new Date(Date.UTC(1971, 0, 1))
]) β "1970-07-02T12:00:00.000Z"
.toISOString() method.dates is an empty array return null.