refactor csv string parameter handling - fix empty string to array split

[jfchevrette]

What this PR does

split when passed an empty string, will return an array with the empty string as the first element, even if there were no occurrences of the split delimiter. In our use case, we actually want no items in the array if there were no items in the string (empty string should be considered no item)

This current breaks the case when no availability zones are passed (either by the use of an empty array or by passing an empty string)

Example showing what happens

var emptyArray = []
var joinEmptyArray = join(emptyArray, ',') // => ""
var splitEmptyString = split(joinEmptyArray, ',') // => [""]

The change improves the param description and adds helper functions which help clarify the intent and makes it reusable

Jira:
Link to demo recording:

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