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:

	Array(4).fill(null).map(...)