refactor: moved null check

Author: Warren-Pitterson

src/ServiceLayer.Mesh/FileTypes/NbssAppointmentEvents/FileParser.cs

@@ -19,13 +19,13 @@ public class FileParser : IFileParser
     /// </summary>
     public ParsedFile Parse(Stream stream)
     {
-        var result = new ParsedFile();
-
         if (stream == null)
         {
             throw new ArgumentNullException(nameof(stream), "Stream cannot be null");
         }
 
+        var result = new ParsedFile();
+
         using var reader = CreateStreamReader(stream);
         using var csv = CreateCsvReader(reader);