Fix NotFound error on Get and comparison

[FabienTschanz]

Pull Request (PR) description

This PR fixes an error when the resource does not exist (because of -ErrorAction Stop instead of SilentlyContinue...) and removes the LastHeartbeatDateTime property from the comparison since its value can't be changed. It's determined by the system.

This Pull Request (PR) fixes the following issues

None.

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.