Handle empty args for get_terraform_cli_args function

[camlow325]

Closes #1214

When the get_terraform_cli_args() function is called and the list of CLI args is empty, the function previously caused Terragrunt to crash. With the change in this MR, the function would now just return an empty list to the caller when the list of CLI args is empty.

[yorinasub17]

This looks good to me. However, before merging, it would be nice to have a regression test that reproduces the error and verifies it works. If you need ideas on how to implement one, check out the TestResolveTerragruntInterpolation unit test.

[camlow325]

Thanks for the review and comments, @yorinasub17. I added a unit test for the fix. Locally, the new test fails with the "must not call ListVal with empty" error without the fix and passes with the fix. Please let me know if you see anything else that you would like for me to update for this. Thanks again!

[yorinasub17]

Thanks for adding a test! LGTM! Will kick off a build and if it passes we can merge this in.

[yorinasub17]

Build passed, so will go ahead and merge + release this. Thanks again for your contribution!

[yorinasub17]

Released as https://github.com/gruntwork-io/terragrunt/releases/tag/v0.23.25 . Binaries will show up shortly.