feat: add new rule to check comparisons with empty list

[bmbferreira]

As explained by @jbardin on this issue "...the preferred way to check for an empty set or series of any type would be to check its length, rather than compare it against a literal value".
I've had a bug on my terraform code that took me almost 2 days to find out the root cause, just to find out that we had a check for [] in our code that was returning always false. 😓
Since we are already using tflint, having a check for it would be a really good way to avoid doing the same stupid mistake again! 😃 Hopefully, you will also find this new rule useful and everyone can save a lot of headaches with it as well.

More info about why this rule is "needed":
https://discuss.hashicorp.com/t/conditional-for-empty-list/26958/2
hashicorp/terraform#25798 (comment)

[bendrucker]

Nice, few minor comments

[bmbferreira]

@bendrucker thank you for reviewing it so quickly! 🙇

Just fixed all the issues reported.

Have a great day!

[bendrucker]

Will get back on this tomorrow

[bmbferreira]

Will get back on this tomorrow

Thank you!

[bmbferreira]

sorry to bother you @bendrucker, but please let me know if there's still something missing on the PR 😃

thank you! and have an awesome week!

[wata727]

It looks good to me. If @bendrucker approves, merge this pull request.

[sergei-ivanov]

Thanks for adding this rule, but unfortunately it only picks up the most trivial cases.

For example, it picks up this:

resource "aws_security_group_rule" "outbound_proxy" {
  count =var.proxy_subnets != [] ? 1 : 0

but not this:

resource "aws_security_group_rule" "outbound_proxy" {
  count = var.enabled && var.proxy_subnets != [] ? 1 : 0

nor this:

templatefile("${path.module}/jenkins.yaml.tmpl", {
    JENKINS_INGRESS_ENABLED = var.jenkins_controller_alb_allow_list != []
})

I guess it needs to implement more recursion for parsing into more complex expressions.

[bmbferreira]

Thanks for the feedback @sergei-ivanov! Can you report this as an issue so I can tackle it when I have some time please?

🙇

[sergei-ivanov]

@bmbferreira I have created #1424 -- thank you very much for your contribution!