fix: fix NRE when configuration is invalid

and so _httpClient is not set
occuring when commit form is opened

Author: pmiossec

src/GitExtensions.AzureDevOpsCommitMessage/Plugin.cs

@@ -322,7 +322,7 @@ public override void Unregister(IGitUICommands gitUiCommands)
 
         private void gitUiCommands_PreCommit(object sender, GitUIEventArgs e)
         {
-            if (!_enabledSettings.ValueOrDefault(Settings))
+            if (_httpClient == null || !_enabledSettings.ValueOrDefault(Settings))
             {
                 return;
             }