[dio] Revert changes to CancelToken.cancel behavior (cfug#1769)

### New Pull Request Checklist

- [x] I have read the
[Documentation](https://pub.dev/documentation/dio/latest/)
- [x] I have searched for a similar pull request in the
[project](https://github.com/cfug/dio/pulls) and found none
- [x] I have updated this branch with the latest `main` branch to avoid
conflicts (via merge from master or rebase)
- [x] I have added the required tests to prove the fix/feature I'm
adding
- [x] I have updated the documentation (if necessary)
- [x] I have run the tests without failures
- [x] I have updated the `CHANGELOG.md` in the corresponding package

### Additional context and info (if any)

Fixes cfug#1765

dio/CHANGELOG.md

@@ -2,7 +2,9 @@
 
 ## Unreleased
 
-*None.*
+- Revert changes to `CancelToken.cancel()` behavior, as a result the `DioError`
+  provided by the `CancelToken.cancelError` does not contain useful information
+  when the token was not used with a request.
 
 ## 5.1.0
 

dio/lib/src/cancel_token.dart

@@ -31,13 +31,8 @@ class CancelToken {
 
   /// Cancel the request with the given [reason].
   void cancel([Object? reason]) {
-    if (requestOptions == null) {
-      throw StateError(
-        'CancelToken was canceled before being used in a request.',
-      );
-    }
     _cancelError = DioError.requestCancelled(
-      requestOptions: requestOptions!,
+      requestOptions: requestOptions ?? RequestOptions(),
       reason: reason,
       stackTrace: StackTrace.current,
     );

dio/test/cancel_token_test.dart

@@ -16,10 +16,8 @@ void main() {
       token.cancel(reason);
     });
 
-    test('cancel without use throws StateError', () async {
-      final token = CancelToken();
-
-      expect(() => token.cancel(), throwsStateError);
+    test('cancel without use does not throw (#1765)', () async {
+      CancelToken().cancel();
     });
   });
 }