fix(misconf): check if property is not nil before conversion

[nikpivkin]

Description

This PR fixes the panic of trying to convert a property that is missing from the template.

Template:

---
AWSTemplateFormatVersion: 2010-09-09
Description: Godd example of excessive ports
Resources: 
  NetworkACL:
    Type: AWS::EC2::NetworkAcl
  Rule:
    Type: AWS::EC2::NetworkAclEntry
    Properties:
      NetworkAclId:
        Ref: NetworkACL

Output:

trivy conf main.yaml
2024-09-23T23:28:45+06:00       INFO    [misconfig] Misconfiguration scanning is enabled
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0xe0 pc=0x10363f0c4]

goroutine 1 [running]:
github.com/aquasecurity/trivy/pkg/iac/scanners/cloudformation/parser.(*Property).Type(...)
        /home/runner/work/trivy/trivy/pkg/iac/scanners/cloudformation/parser/property.go:108
github.com/aquasecurity/trivy/pkg/iac/scanners/cloudformation/parser.(*Property).ConvertTo(0x0, {0x1053ac0f7, 0x3})
        /home/runner/work/trivy/trivy/pkg/iac/scanners/cloudformation/parser/property_conversion.go:66 +0x24
github.com/aquasecurity/trivy/pkg/iac/adapters/cloudformation/aws/ec2.getRules({0x1400491f8e0, 0xa}, {{0x1400491f810, 0x9}, {0x140039bd080, 0xb, 0xb}, {0x1053addf7, 0x4}, {0x0, ...}, ...})
        /home/runner/work/trivy/trivy/pkg/iac/adapters/cloudformation/aws/ec2/nacl.go:68 +0x1638

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).