Switch to c# 8 with nullable reference types

[SimonGeering]

Is your feature request related to a problem? Please describe.
The root cause of #33 is potentially a null reference exception.

Describe the solution you'd like
To prevent null reference exceptions we should switch all projects to use c#8 language features with nullable reference types:

    <LangVersion>8.0</LangVersion>
    <nullable>enable</nullable>

Describe alternatives you've considered

• lots of guard clauses
• lots of extra unit tests
• lots of frustration :-)

Additional context
I have disabled unit test that are causing this issue for now in the PR for #9

[SimonGeering]

Happy to do a PR for this if you want to assign it to me @benrick .