Fixes default value apply of JournalingReportNdrTo in EXO Transport Config

[FabienTschanz]

Pull Request (PR) description

This PR fixes an issue where the default value <> of JournalingReportNdrTo in the EXOTransportConfig could be applied to the tenant, which will throw an exception. If the default value or an empty string is present, the property won't be applied because it has to be a valid email address. Once set, the setting cannot be removed.

This Pull Request (PR) fixes the following issues

• Fixes BUG:EXOTransportConfig :System.InvalidOperationException|Before you create a journal rule #5606

Task list

• Added an entry to the change log under the Unreleased section of the file CHANGELOG.md.
  Entry should say what was changed and how that affects users (if applicable), and
  reference the issue being resolved (if applicable).
• Resource parameter descriptions added/updated in the schema.mof.
• Resource documentation added/updated in README.md.
• Resource settings.json file contains all required permissions.
• Examples appropriately added/updated.
• Unit tests added/updated.
• New/changed code adheres to DSC Community Style Guidelines.

[ricmestre]

LGTM 🚀