Omitempty-style handling for array elements

Suppose you have an argument that's a `[String]!` and you want to pass `["a", null, "c"]`. Right now you pretty much have to do `pointer: true`, then pass `[]*string{&"a", null, &"c"}` (except of course that's not valid syntax, the real syntax is uglier until golang/go#45624). But we should be able to just let you apply the `omitempty` to the elements, instead of or in addition to to the whole array, so that you can pass `[]string{"a", "", "c"}` and it does the thing you mean. It may need a name since it's really "translate empty elements to null", rather than omitting them.

See also #16 and #14.

Start by comparing the requested behavior with related issues #16 and #14, then trace how omitempty is currently applied to array arguments. Define how empty string elements should become null without omitting the array, including the naming decision, and verify the behavior for ["a", "", "c"].

Written by the indexing model from the issue text.