xADComputer: Fix ServicePrincipalNames Empty String Exception

[X-Guardian]

Pull Request (PR) description

This PR fixes the Exception that occurs when the ServicePrincipalNames property is set to an empty string and is not in the desired state.

Additional tests have been added to test all properties being set to empty strings.

This Pull Request (PR) fixes the following issues

• Fixes xADComputer: Exception When Setting ServicePrincipalNames to An Empty String #382

Task list

• Added an entry under the Unreleased section in the CHANGELOG.md.
  Entry should say what was changed, and how that affects users (if applicable).
• Resource documentation added/updated in resource directory README.md.
• 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.
• New/changed code adheres to DSC Resource Style Guidelines and Best Practices.

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This change is 

[codecov-io]

Codecov Report

Merging #388 into dev will not change coverage.
The diff coverage is n/a.

@@         Coverage Diff         @@
##            dev   #388   +/-   ##
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  Coverage    92%    92%           
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  Files        20     20           
  Lines      2530   2530           
  Branches     10     10           
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  Hits       2335   2335           
  Misses      185    185           
  Partials     10     10

[johlju]

Reviewed 2 of 2 files at r1, 1 of 1 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved