Fix Null Reference on Content-Saftey service (microsoft#222)

### Motivation and Context

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Reported by customer in
microsoft#218

### Description

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Use null-coalesce operator to return false when service null

### Contribution Checklist

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- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [Contribution
Guidelines](https://github.com/microsoft/copilot-chat/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
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raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

webapi/Controllers/DocumentImportController.cs

@@ -81,7 +81,7 @@ private enum SupportedFileType
     private const string ReceiveMessageClientCall = "ReceiveMessage";
     private readonly IOcrEngine _ocrEngine;
     private readonly IAuthInfo _authInfo;
-    private readonly IContentSafetyService? _contentSafetyService = null;
+    private readonly IContentSafetyService? _contentSafetyService;
 
     /// <summary>
     /// Initializes a new instance of the <see cref="DocumentImportController"/> class.
@@ -119,7 +119,7 @@ public DocumentImportController(
     [ProducesResponseType(StatusCodes.Status200OK)]
     public bool ContentSafetyStatus()
     {
-        return this._contentSafetyService!.ContentSafetyStatus(this._logger);
+        return this._contentSafetyService?.ContentSafetyStatus(this._logger) ?? false;
     }
 
     /// <summary>
@@ -298,7 +298,7 @@ private async Task ValidateDocumentImportFormAsync(DocumentImportForm documentIm
                     {
                         if (documentImportForm.UseContentSafety)
                         {
-                            if (!this._contentSafetyService!.ContentSafetyStatus(this._logger))
+                            if (this._contentSafetyService == null || !this._contentSafetyService.ContentSafetyStatus(this._logger))
                             {
                                 throw new ArgumentException("Unable to analyze image. Content Safety is currently disabled in the backend.");
                             }
@@ -307,8 +307,8 @@ private async Task ValidateDocumentImportFormAsync(DocumentImportForm documentIm
                             try
                             {
                                 // Call the content safety controller to analyze the image
-                                var imageAnalysisResponse = await this._contentSafetyService!.ImageAnalysisAsync(formFile, default);
-                                violations = this._contentSafetyService.ParseViolatedCategories(imageAnalysisResponse, this._contentSafetyService!.Options!.ViolationThreshold);
+                                var imageAnalysisResponse = await this._contentSafetyService.ImageAnalysisAsync(formFile, default);
+                                violations = this._contentSafetyService.ParseViolatedCategories(imageAnalysisResponse, this._contentSafetyService.Options.ViolationThreshold);
                             }
                             catch (Exception ex) when (!ex.IsCriticalException())
                             {