Optional strings should default to null not ""

[cassamajor]

Describe the bug

When I ran terraform-docs tfvars hcl ., optional strings with no default value were represented as "".

I did a bit of reading on Type Constraints on Optional Object Type Attributes. Long story short is, a string type with no default value evaluates to tostring(null), not "".

How can we reproduce it?

1. Clone this repository
2. cd into the examples/terraform/aws directory.
3. Run terraform-docs tfvars hcl .

Environment information

terraform-docs version v0.17.0 0.17.0 darwin/amd64