Tool throws a panic error when there is a null type in the ternary operator

[cybersa]

• terrascan version: v1.18.3
• Operating System: Linux

Description

terrascan fails with panic error when I run it against the following terraform script.

resource "null_resource" "cluster" {
  provisioner "local-exec" {
    command = "echo '${var.rds_create_db ? "DBNAME" : null}' >> dbs.txt"
  }
}

Tool fails because of null type in the ternary operator. If I replace that null value with literal string "null" or something else, it's works fine.

What I Did

terrascan scan -f test.tf
panic: value is null

goroutine 1 [running]:
github.com/zclconf/go-cty/cty.Value.AsString({{{0x37398a8?, 0xc00081c75d?}}, {0x0?, 0x0?}})
	/home/runner/go/pkg/mod/github.com/zclconf/go-cty@v1.10.0/cty/value_ops.go:1259 +0x125
github.com/tenable/terrascan/pkg/iac-providers/terraform/commons.(*converter).convertStringPart(0xc00099eca0, {0x373a760?, 0xc0006ec480?})
	/home/runner/work/terrascan/terrascan/pkg/iac-providers/terraform/commons/convert.go:226 +0x109
github.com/tenable/terrascan/pkg/iac-providers/terraform/commons.(*converter).convertTemplateConditional(0xc00099eca0, 0xc0004a7c70)
	/home/runner/work/terrascan/terrascan/pkg/iac-providers/terraform/commons/convert.go:262 +0x456
github.com/tenable/terrascan/pkg/iac-providers/terraform/commons.(*converter).convertStringPart(0xc00099eca0, {0x373a660?, 0xc0004a7c70?})
	/home/runner/work/terrascan/terrascan/pkg/iac-providers/terraform/commons/convert.go:232 +0x185
github.com/tenable/terrascan/pkg/iac-providers/terraform/commons.(*converter).convertTemplate(0x1?, 0x1?)
	/home/runner/work/terrascan/terrascan/pkg/iac-providers/terraform/commons/convert.go:210 +0x107
github.com/tenable/terrascan/pkg/iac-providers/terraform/commons.(*converter).convertExpression(0xc00099eca0, {0x373a920, 0xc0006ec7e0})
	/home/runner/work/terrascan/terrascan/pkg/iac-providers/terraform/commons/convert.go:166 +0x272
github.com/tenable/terrascan/pkg/iac-providers/terraform/commons.(*converter).convertBody(0x203000?, 0xc000654bb0)
	/home/runner/work/terrascan/terrascan/pkg/iac-providers/terraform/commons/convert.go:56 +0xe7
github.com/tenable/terrascan/pkg/iac-providers/terraform/commons.(*converter).convertBlock(0x8?, 0xc0004385a0, 0xc00099e8e8, 0xc00099e8b8)
	/home/runner/work/terrascan/terrascan/pkg/iac-providers/terraform/commons/convert.go:97 +0x65
github.com/tenable/terrascan/pkg/iac-providers/terraform/commons.(*converter).convertBody(0xc00044cc90?, 0xc000654fd0)
	/home/runner/work/terrascan/terrascan/pkg/iac-providers/terraform/commons/convert.go:70 +0x314
github.com/tenable/terrascan/pkg/iac-providers/terraform/commons.CreateResourceConfig(_)
	/home/runner/work/terrascan/terrascan/pkg/iac-providers/terraform/commons/resource.go:48 +0x125
github.com/tenable/terrascan/pkg/iac-providers/terraform/commons.LoadIacFile({0xc00044cc90, 0x22}, {0x305c94c, 0x6})
	/home/runner/work/terrascan/terrascan/pkg/iac-providers/terraform/commons/load-file.go:58 +0x499
github.com/tenable/terrascan/pkg/iac-providers/terraform/v15.(*TfV15).LoadIacFile(0x29da420, {0xc00044cc90?, 0x3068455?}, 0xb?)
	/home/runner/work/terrascan/terrascan/pkg/iac-providers/terraform/v15/load-file.go:26 +0x30
github.com/tenable/terrascan/pkg/runtime.(*Executor).Execute(0xc000928a80, 0x0, 0x0)
	/home/runner/work/terrascan/terrascan/pkg/runtime/executor.go:226 +0x1d9
github.com/tenable/terrascan/pkg/cli.(*ScanOptions).Run(0xc000581520)
	/home/runner/work/terrascan/terrascan/pkg/cli/run.go:218 +0x44a
github.com/tenable/terrascan/pkg/cli.(*ScanOptions).Scan(0x0?)
	/home/runner/work/terrascan/terrascan/pkg/cli/run.go:140 +0x1b7
github.com/tenable/terrascan/pkg/cli.scan(0x4ea5300?, {0xc0003fa540?, 0x2?, 0x2?})
	/home/runner/work/terrascan/terrascan/pkg/cli/scan.go:55 +0x11f
github.com/spf13/cobra.(*Command).execute(0x4ea5300, {0xc0003fa520, 0x2, 0x2})
	/home/runner/go/pkg/mod/github.com/spf13/cobra@v1.6.1/command.go:916 +0x862
github.com/spf13/cobra.(*Command).ExecuteC(0x4ea5020)
	/home/runner/go/pkg/mod/github.com/spf13/cobra@v1.6.1/command.go:1044 +0x3bd
github.com/spf13/cobra.(*Command).Execute(...)
	/home/runner/go/pkg/mod/github.com/spf13/cobra@v1.6.1/command.go:968
github.com/tenable/terrascan/pkg/cli.Execute()
	/home/runner/work/terrascan/terrascan/pkg/cli/register.go:98 +0x435
main.main()
	/home/runner/work/terrascan/terrascan/cmd/terrascan/main.go:22 +0x17