Does not use argument's default value when null is passed

[stigok]

Terraform Version

Terraform v0.14.6
+ provider registry.terraform.io/hashicorp/null v3.0.0
+ provider registry.terraform.io/hashicorp/random v3.0.1

Terraform Configuration Files

https://github.com/stigok/terraform-use-default-value-when-null

Debug Output

https://raw.githubusercontent.com/stigok/terraform-use-default-value-when-null/main/tfdebug.log

Crash Output

Expected Behavior

The modules/test module should use the module's default value for my_var when it is passed a null value, as stated in the Terraform syntax documentation:

null: a value that represents absence or omission. If you set an argument of a resource or module to null, Terraform behaves as though you had completely omitted it — it will use the argument's default value if it has one, or raise an error if the argument is mandatory. null is most useful in conditional expressions, so you can dynamically omit an argument if a condition isn't met.

Actual Behavior

my_var in the resource definition is not ignored as if it was ommited, but passed as an acutal null value, which in this example breaks string interpolation.

Steps to Reproduce

1. terraform init
2. terraform plan

Additional Context

Linux 64bit

References

Somewhat related to #5471

[jbardin]

Thanks @stigok

We're tracking this in #24142

[ghost]

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