Fix/tfs attachment enricher unable to export (#2008)

From: @jacobmalliet

Resolves an issue with the `TfsAttachmentEnricher` where `_server`
instance was always `null`. This seemingly occurred because of a
duplicate `WorkItemServer` instance variable `_workItemServer`.

After attempting to use `workItemServer` instead, I found that it was
also not being set properly because of a null comparison being done
incorrectly.

This change removes the duplicate `WorkItemServer` variable and properly
initializes the `_workItemServer` before exporting attachments.

First time contributing so let me know if you need anything else changed
here.

src/MigrationTools.Clients.AzureDevops.ObjectModel/ProcessorEnrichers/TfsAttachmentEnricher.cs

@@ -19,7 +19,6 @@ namespace MigrationTools.ProcessorEnrichers
 {
  public class TfsAttachmentEnricher : WorkItemProcessorEnricher, IAttachmentMigrationEnricher
  {
- private WorkItemServer _server;
  private string _exportWiPath;
  private TfsAttachmentEnricherOptions _options; 
  private WorkItemServer _workItemServer;
@@ -116,7 +115,7 @@ private string ExportAttachment(WorkItem wi, Attachment wia, string exportpath)
  Log.LogDebug(string.Format("...downloading {0} to {1}", fname, exportpath));
  try
  {
- var fileLocation = _server.DownloadFile(wia.Id);
+ var fileLocation = _workItemServer.DownloadFile(wia.Id);
  File.Copy(fileLocation, fpath, true);
  }
  catch (Exception ex)
@@ -178,7 +177,7 @@ public string GetSafeFilename(string filename)
 
  private void SetupWorkItemServer()
  {
- if (_workItemServer != null)
+ if (_workItemServer == null)
  {
  IMigrationEngine engine = Services.GetRequiredService<IMigrationEngine>();
  _workItemServer = engine.Source.GetService<WorkItemServer>();