sysctl-34 - fs.protected_hardlinks and fs.protected_symlinks can be nil

The sysctl-34 checks are currently failing with the latest amzn linux 2 images from aws (used ami filter amzn2-ami-hvm-*-x86_64-gp2):

CIS-AMZN2.amazon-ebs.amz-ami:   ×  sysctl-34: Ensure links are protected (2 failed)
CIS-AMZN2.amazon-ebs.amz-ami:      ✔  Kernel Parameter fs.protected_fifos value is expected to eq 1 or eq 2 or eq nil
CIS-AMZN2.amazon-ebs.amz-ami:      ×  Kernel Parameter fs.protected_hardlinks value is expected to eq 1
CIS-AMZN2.amazon-ebs.amz-ami:
CIS-AMZN2.amazon-ebs.amz-ami:      expected: 1
CIS-AMZN2.amazon-ebs.amz-ami:           got: nil
CIS-AMZN2.amazon-ebs.amz-ami:
CIS-AMZN2.amazon-ebs.amz-ami:      (compared using ==)
CIS-AMZN2.amazon-ebs.amz-ami:
CIS-AMZN2.amazon-ebs.amz-ami:      ✔  Kernel Parameter fs.protected_regular value is expected to eq 2 or eq nil
CIS-AMZN2.amazon-ebs.amz-ami:      ×  Kernel Parameter fs.protected_symlinks value is expected to eq 1
CIS-AMZN2.amazon-ebs.amz-ami:
CIS-AMZN2.amazon-ebs.amz-ami:      expected: 1
CIS-AMZN2.amazon-ebs.amz-ami:           got: nil

The issue triggered by the following lines.

fs.protected_hardlinks:
https://github.com/dev-sec/linux-baseline/blob/81ce2ab60cc4bc29aad3822897de1c55593e3f73/controls/sysctl_spec.rb#L420

fs.protected_symlinks:
https://github.com/dev-sec/linux-baseline/blob/81ce2ab60cc4bc29aad3822897de1c55593e3f73/controls/sysctl_spec.rb#L426

A possible fix is similar like the already implemented exceptions for fs.protected_regular and fs.protected_fifos:

  describe kernel_parameter('fs.protected_hardlinks') do # include nil because amzn linux does not have this parameter
    its(:value) { should eq(1).or eq(nil) }
  end

  describe kernel_parameter('fs.protected_symlinks') do
    its(:value) { should eq(1).or eq(nil) } # include nil because amzn linux does not have this parameter
  end

Start in controls/sysctl_spec.rb at the kernel_parameter checks around lines 420 and 426, and compare them with the existing fs.protected_regular and fs.protected_fifos expectations. Verify the checks handle Amazon Linux 2 values as described, then run the relevant InSpec profile tests and confirm sysctl-34 passes without weakening the required values on systems that provide them.