fix(terraform): add null as allowed value, when checking for presence of public IP in CKV2_AZURE_39

[phoehnel]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Description

Currently, the CKV2_AZURE_39 fails, when the value of public_ip_address_id inside of azurerm_network_interface is set to "null" which has the same effect as the length and existence operators already in place.

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] I have added tests that prove my feature, policy, or fix is effective and works
• [x] New and existing tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules