find_unused fails with '_pyjq.ScriptRuntimeError: Cannot iterate over null (null)'

I've tried this in both linux (WSL2 under Win10 in case it makes any difference) as well as Docker with the same results.
The `collect` command runs successfully. I then run the `find_unused` command which fails with the following stack trace:

```
(venv) me@me:/mnt/c/Projects/cloudmapper$ python cloudmapper.py find_unused --accounts myaccount
Traceback (most recent call last):
File "cloudmapper.py", line 72, in
main()
File "cloudmapper.py", line 66, in main
commands[command].run(arguments)
File "/mnt/c/Projects/cloudmapper/commands/find_unused.py", line 13, in run
unused_resources = find_unused_resources(accounts)
File "/mnt/c/Projects/cloudmapper/shared/find_unused.py", line 115, in find_unused_resources
find_unused_security_groups(region),
File "/mnt/c/Projects/cloudmapper/shared/find_unused.py", line 21, in find_unused_security_groups
for sg in pyjq.all(".SecurityGroups[]", defined_sgs):
File "/mnt/c/Projects/cloudmapper/venv/lib/python3.8/site-packages/pyjq.py", line 49, in all
return compile(script, vars, library_paths).all(_get_value(value, url, opener))
File "_pyjq.pyx", line 209, in _pyjq.Script.all (_pyjq.c:2561)
_pyjq.ScriptRuntimeError: Cannot iterate over null (null)
```

I get the same error and stack trace from the Docker instance as well. I did see from #617 that the `find_unused` command is barely used, but the main reason I've been looking into this sort of application is to help identify unused resources from a cost management perspective rather than security audit. So for me, finding orphaned resources that don't link back to running instances is the primary goal.

NB: the `collect` command does return some errors... it doesn't look like those are related to the `find_unused` issue at all, but including them just in case:
```
Summary: 1415 APIs called. 4 errors
Failures:
s3.get_bucket_acl({'Bucket': 'elasticbeanstalk-af-south-1-988217567069'}): An error occurred (IllegalLocationConstraintException) when calling the GetBucketAcl operation: The af-south-1 location constraint is incompatible for the region specific endpoint this request was sent to.
s3.get_bucket_policy({'Bucket': 'elasticbeanstalk-af-south-1-988217567069'}): An error occurred (IllegalLocationConstraintException) when calling the GetBucketPolicy operation: The af-south-1 location constraint is incompatible for the region specific endpoint this request was sent to.
s3.get_bucket_logging({'Bucket': 'elasticbeanstalk-af-south-1-988217567069'}): An error occurred (IllegalLocationConstraintException) when calling the GetBucketLogging operation: The af-south-1 location constraint is incompatible for the region specific endpoint this request was sent to.
s3.get_bucket_encryption({'Bucket': 'elasticbeanstalk-af-south-1-988217567069'}): An error occurred (IllegalLocationConstraintException) when calling the GetBucketEncryption operation: The af-south-1 location constraint is incompatible for the region specific endpoint this request was sent to.
```