Fixes 'Key Vault Role Based Access Control Disabled' check

[rieck-srlabs]

Description

ScoutSuite failed to flag key vaults where the enable_rbac_authorization field was set to null. Through manual configuration in the Azure portal I confirmed that RBAC Access Control is disabled if this field is set to null.

Fixes #1607

Type of change

Select the relevant option(s):

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

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] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works (optional)
• [x] New and existing unit tests pass locally with my changes