(build) Handle null value for PreReleaseLabel

Depending on what branch is in play, the PreReleaseLabel may not
contain a value, as such, we need to handle when this hasn't been set,
and only attempt to grab a prerelease value when there is known to be
one.

BuildScripts/default.ps1

@@ -67,7 +67,9 @@ task Run-GitVersion {
     # Having a pre-release label of greater than 10 characters can cause problems when trying to run choco pack.
     # Since we typically only see this when building a local feature branch, or a PR, let's just trim it down to
     # the 10 character limit, and move on.
-    $prerelease = $versionInfo.PreReleaseLabel.Replace("-","").Substring(0,10)
+    if ($versionInfo.PreReleaseLabel) {
+        $prerelease = $versionInfo.PreReleaseLabel.Replace("-","").Substring(0,10)
+    }
 
     # Chocolatey doesn't support a prerelease that starts with a digit.
     # If we see a digit here, merely replace it with an `a` to get around this.