jq: error (at <stdin>:0): Cannot iterate over null (null) - jq

Question

I've been working with an API call to structure it in JSON format so I might later push it into a database. Then code looks like this:
getPage() {
curl --fail -X GET 'https://api.app.com/v1/test?page=1&pageSize=1000&sort=desc' \
-H 'Authorization: Bearer 123abc456pickupsticks789' \
-H 'cache-control: no-cache'
}
getPage \
| jq -c '.items | .[] | {landing_id: .landing_id, submitted_at: .submitted_at, answers: .answers, email: .hidden.email}' \
> testpush.json
When I run it though, it produces this error: jq: error (at <stdin>:0): Cannot iterate over null (null)
I've looked at solutions such as this one, or this one from this site, and this response.
The common solution seemed to be using a ? in front of [] and I tried it in the jq line towards the bottom, but it still does not work. It just produces an empty json file.
Am I misreading the takeaway from those other answers and not putting my ? in the right place?>

Answer 1

To protect against the possibility that .items is not an array, you could write:
.items | .[]?
or even more robustly:
try .items[]
which is equivalent to (.items[])?.
In summary:
try E is equivalent to try E catch empty
try E is equivalent to (E)?
(Note that the expressions .items[]? and (.items[])? are not identical.)
However none of these will provide protection against input that is invalid JSON.
p.s. In future, please follow the mcve guidelines (http://stackoverflow.com/help/mcve); in the present case, it would have helped if you had provided an illustrative JSON snippet based on the output produced by the curl command.

Answer 2

It is necessary to let JSON know that it can continue after an unexpected value while parsing that array. try or ? are perfect options for that.
Bear in mind that it is either necessary to guarantee the data or let the interpreter to know that it is ok to continue. It may sounds redundant, but it is something like a fail-safe approach to prevent unexpected results that are harder to track/notice.
Also, it is necessary to be aware about the differences for "testing" between ? vs try.
Assuming that $sample meets JSON standards the code bellow will work always:
sample='{"myvar":1,"var2":"foo"}'
jq '{newVar: ((.op[]? | .item) // 0)}' <<< $sample
so, the op array is null for $sample as above, but it is clear to jq that it can continue without asking for your intervention/fix.
But if you do assume ? as the same as try, you may get an error (took me a loot to learn this, and it is not clear in the documentation). As an example of improper use of ? we have:
sample='{"myvar":1,"var2":"foo"}'
jq '{newVar: (.op[].item? // 0)}' <<< $sample
So, as op is null it will lead to an error, because you are telling to jq to ignore an error while retrieving .item, while there is mention about the possibility of an error during the attempt to iterate over null (in this case .op[]), and that attempt happened before that point checking for .item.
On the other hand, try would work in this case:
sample='{"myvar":1,"var2":"foo"}'
jq '{newVar: (try .op[].item catch 0)}' <<< $sample
This is a small use difference that can lead to a large difference in the result