add list with an empty string, necessary for terraform 0.12 destroy

[schollii]

what

Add the missing empty string list to coalesce() call

I have not tested whether this breaks terraform 0.11 destroy, but based on what I read it should not.

why

• without this fix, issue terraform destroy fails due to empty coalesce in output #51 arises for terraform 0.12 destroy

references

• coalescelist behavior with all empty lists changed in v0.12.0 hashicorp/terraform#21370
• Setting instance_count = 0 leads to error with Terrafrom 0.12 terraform-aws-modules/terraform-aws-ec2-instance#104
• closes #51

[Gowiem]

/test all

[Gowiem]

@schollii Huh, I believe I've run into this issue before, but did not know it was a recurring problem within HCL... interesting. Have you found a Terraform issue that calls this out? Seems like it should be dealt with upstream, though I get why we'd want to fix it here.

Anyway, approved!

[Gowiem]

@schollii Thanks for the contribution! Released as 0.18.2.

Also, saw the issue you linked -- bummer that got closed... should resurface that issue with the Terraform team so this isn't needed. They should fix it, I'm sure it wouldn't be a big fix.