Make ServiceInvoker treat Nullable<T> parameters as a match for null arguments

[khellang]

Background

When passing null to an async service method accepting a Nullable<T> parameter, the call fails as Halibut will not consider the method a match.

Results

Fixes #603

Before

Passing null to a service method with a Nullable<T> parameter failed. See #603

After

Invoking the method no longer fails.

Pre-requisites

• [x] I have read How we use GitHub Issues for help deciding when and where it's appropriate to make an issue.
• [x] I have considered informing or consulting the right people, according to the ownership map.
• [x] I have considered appropriate testing for my change.

[CLAassistant]

All committers have signed the CLA.

[khellang]

Hello good folks! 👋 Any hint on when we can get this looked at? We've worked around it by splitting the RPC interface with different overloads with and without the nullable argument, but would really like a proper fix 😅