Use else if() clause

Author: MaggieKimani1

src/Microsoft.OpenApi.Hidi/OpenApiService.cs

@@ -581,7 +581,7 @@ private static string GetInputPathExtension(string openapi = null, string csdl =
             {
                 extension = Path.GetExtension(openapi);
             }
-            if (!string.IsNullOrEmpty(csdl))
+            else if (!string.IsNullOrEmpty(csdl))
             {
                 extension = ".yml";
             }