json-output: Fix unknowns for tuples and sets

[alisdair]

The JSON output for sequences previously omitted unknown values for tuples and sets, which made it impossible to interpret the corresponding unknown marks. For example, consider this resource:

resource "example_resource" "example" {
  tags = toset(["alpha", timestamp(), "charlie"])
}

This would previously be encoded in JSON as:

"after": {
    "tags": ["alpha", "charlie"]
},
"after_unknown": {
    "id": true,
    "tags": [false, true, false]
},

That is, the timestamp value would be omitted from the output altogether, while the corresponding unknown marks would include a value for each of the set members.

This commit changes the behaviour to:

"after": {
    "tags": ["alpha", null, "charlie"]
},
"after_unknown": {
    "id": true,
    "tags": [false, true, false]
},

This aligns tuples and sets with the prior behaviour for lists, and makes it clear which elements are known and which are unknown.

I consider this a bug fix, as the prior output was not usable, and I believe this behaviour is correct.

[cam-stitt]

Makes sense!

[github-actions]

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[github-actions]

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