Variable with `default = null` are not handled properly

[flesko-profinit]

Issue Description

The VariableRenderer in Checkov does not correctly handle variables with a null default. When a variable is defined with default = null and no value is passed from outside the module, the renderer should remove the attribute from the parsed configuration, rather than leaving it unresolved as var.<name>.

This likely requires a change in renderer.py originates from the default and should be treated as an absence of the attribute.

Examples

variable "key" {
  type = string
  default = 'key'
}

resource "aws_cloudwatch_log_group" "pass" {
  retention_in_days = 1
  kms_key_id        = var.key
}

Gives conf:

{'__end_line__': 18, '__start_line__': 15, 'kms_key_id': ['key'], 'retention_in_days': [1], '__address__': 'aws_cloudwatch_log_group.pass'}

While

variable "key" {
  type = string
  default = null
}

resource "aws_cloudwatch_log_group" "pass" {
  retention_in_days = 1
  kms_key_id        = var.key
}

Gives conf:

{'__end_line__': 18, '__start_line__': 15, 'kms_key_id': ['var.key'], 'retention_in_days': [1], '__address__': 'aws_cloudwatch_log_group.pass'}

Expected behaviour is to remove kms_key_id field:

{'__end_line__': 18, '__start_line__': 15, 'retention_in_days': [1], '__address__': 'aws_cloudwatch_log_group.pass'}