💡 Improve DioException.requestOptions explanation when it's empty (#…

…1960)

Address #1943 (comment).

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- [ ] I have searched for a similar pull request in the
[project](https://github.com/cfug/dio/pulls) and found none
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### Additional context and info (if any)

After some investigation, I think there are two other ways we can
resolve this:
1. Apply the latest request options in each interceptor handler, because
the options can be modified in any of them.
2. Treat the field as nullable, but this may be worse and I remember
that we tried once without any good results.

dio/lib/src/dio_exception.dart

@@ -162,6 +162,9 @@ class DioException implements Exception {
       );
 
   /// The request info for the request that throws exception.
+  ///
+  /// The info can be empty (e.g. `uri` equals to "")
+  /// if the request was never submitted.
   final RequestOptions requestOptions;
 
   /// Response info, it may be `null` if the request can't reach to the