Set-GitHubRepositoryTopic: Fix Exception When Clear 'Parameter' is Sp…

…ecified (#216)

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

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

The problem with the existing implementation was that it was creating an array of 1 element, where the element was `$null`.

Fixes #215

GitHubRepositories.ps1

@@ -1033,12 +1033,18 @@ function Set-GitHubRepositoryTopic
         'Clear' = $PSBoundParameters.ContainsKey('Clear')
     }
 
-    $description = "Replacing topics in $RepositoryName"
-    if ($Clear) { $description = "Clearing topics in $RepositoryName" }
+    if ($Clear)
+    {
+        $description = "Clearing topics in $RepositoryName"
+        $Name = @()
+    }
+    else
+    {
+        $description = "Replacing topics in $RepositoryName"
+    }
 
-    $names = @($Name)
     $hashBody = @{
-        'names' = $names
+        'names' = $Name
     }
 
     $params = @{