mark ErrorException as nullable to match interface

[mhornbacher]

Description

This resolves issue #1547 and related issues

Purpose

This pull request is a:

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added tests that prove my fix is effective or that my feature works (no additional tests needed)
• I have added necessary documentation (matches existing documentation)

[mhornbacher]

Hu, it seems that the test suite failed to build the project, dotnet test runs fine locally...

[alexeyzimarev]

@mhornbacher thanks 👍

Some tests are inconclusive due to the build agent specifics, I guess.

[mhornbacher]

Hu, not so happy merging to dev/main a commit where the test suite does not run. I wonder if it has to do with using C# 8 nullable on non-interface code.

[mhornbacher]

Hu, it would seem that other PR's are having the exact same issue: https://github.com/restsharp/RestSharp/pull/1546/checks?check_run_id=1588074709