Avoid returning null in static resource mapper Task

(cherry picked from commit a1ea7accb32bc72f61ed4531d109f76fad843939)

src/Readarr.Http/Frontend/Mappers/StaticResourceMapperBase.cs

@@ -49,7 +49,7 @@ namespace Readarr.Http.Frontend.Mappers
 
             _logger.Warn("File {0} not found", filePath);
 
-            return null;
+            return Task.FromResult<IActionResult>(null);
         }
 
         protected virtual Stream GetContentStream(string filePath)