Set-GitHubRepositoryTopic: Fix Exception When Clear 'Parameter' is Specified

[Simon Heather (X-Guardian)]

Description

This PR fixes the exception in the Set-GithubRepositoryTopic function when the Clear parameter is specified.

Issues Fixed

• FixesSet-GitHubRepositoryTopic: Exception When 'Clear' Parameter is Specified #215

References

https://developer.github.com/v3/repos/#replace-all-repository-topics - Send an empty array ([]) to clear all topics from the repository.

Checklist

• You actually ran the code that you just wrote, especially if you did just "one last quick change".
• Comment-based help added/updated, including examples.
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  is reporting back clean.
• New/changed code adheres to our coding guidelines.
• Changes to the manifest file follow the manifest guidance.
• Unit tests were added/updated and are all passing. See testing guidelines.
• Relevant usage examples have been added/updated in USAGE.md.
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[Howard Wolosky (HowardWolosky)]

Super minor formatting change request, and then this is good to go.
Thanks so much for jumping on this fix so quickly! Glad we have a UT coming in via #176 for this.