Describe the bug

If you create a placeholder module with nothing in the terraform files and create a readme.md file pre-commit will fail with exit 1.
i.e. main.tf, outputs.tf, and variables.tf

We noticed this after creating a placeholder module we want to do work on but not necessarily in a particular PR.

Original creator: @cdobbyn
Moved from antonbabenko/pre-commit-terraform#166

How can we reproduce it?

mkdir bug
cd bug
touch README.md  main.tf  outputs.tf  variables.tf
terraform-docs markdown table .

Environment information

• terraform-docs version: terraform-docs version v0.15.0 f3b6eab linux/amd64
• OS: Windows 10, WSL2 with Ubuntu 20.04.
  Linux DESKTOP-C7315EF 5.4.72-microsoft-standard-WSL2 #1 SMP Wed Oct 28 23:40:43 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux