VM encryptionAtHost set to null if not true

[SeSeicht]

Error Message:

Status Message: The property
| 'securityProfile.encryptionAtHost' is not valid because the
| 'Microsoft.Compute/EncryptionAtHost' feature is not enabled
| for this subscription. (Code:InvalidParameter)

If this feature is not enabled in the subscription, the parameter needs to be empty.
Then the parameter.json can be look like this:
"encryptionAtHost": { "value": false }

related:
#1048

Type of Change

Please delete options that are not relevant.

• 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 (Wiki)

Checklist

• I'm sure there are no other open Pull Requests for the same update/change
• My corresponding pipelines / checks run clean and green without any errors or warnings
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (readme)
• I did format my code

[rahalan]

Please add a successful pipeline run into the PR so we can be sure, the new change is deployable. Thanks.