ScheduledTask: Remove comparison of StartTime if StartTime is not provided

[jensotto]

Pull Request (PR) description

Skip testing for StartTime when no StartTime is specified and ScheduleType is Once, Daily, or Weekly.

This Pull Request (PR) fixes the following issues

- Fixes #148

Task list

• [x] Added an entry under the Unreleased section of the change log in the CHANGELOG.md. Entry should say what was changed, and how that affects users (if applicable).
• [ ] Resource documentation added/updated in README.md in resource folder.
• [ ] Resource parameter descriptions added/updated in schema.mof and comment-based help.
• [ ] Comment-based help added/updated.
• [ ] Localization strings added/updated in all localization files as appropriate.
• [ ] Examples appropriately added/updated.
• [ ] Unit tests added/updated. See DSC Resource Testing Guidelines.
• [ ] Integration tests added/updated (where possible). See DSC Resource Testing Guidelines.
• [x] New/changed code adheres to DSC Resource Style Guidelines and Best Practices.

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This change is 

[PlagueHO]

Hi @jensotto - thanks for submitting this. It looks like the HQRM tests are failing because an entry hasn't been added to the CHANGELOG.md: See the Update the Changelog section in: https://dsccommunity.org/guidelines/contributing/

[jensotto]

Sorry @PlagueHO, I forgot to push that commit.