Fix error when creating virtual network w/o comment

[vavsab]

Description

Comment field is required. At least empty string.

API docs are misleading

Has your change been tested?

Creation is covered with unit tests.

Screenshots (if appropriate):

Types of changes

What sort of change does your code introduce/modify?

• 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 change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• This change is using publicly documented (api.cloudflare.com or developers.cloudflare.com) and stable APIs.

Related issues:
cloudflare/terraform-provider-cloudflare#1602