generate_id: Use Array.fill(null) to initialize an array of null values
When I first wrote this code I reached for a construct of:
Array(4).map(...)
but I found that map doesn't work on an array of empty items like
this. To get things to work I instead used:
[null, null, null, null].map(...)
which did the trick, but only because I happened to be using a
sufficiently small size that it was reasonable to type the complete
literal. For this commit I've found a cleaner approach of: