GenFacades fails to generate not supported assembly for reference source that contains throw null expression body

- [ ] This issue is blocking
- [ ] This issue is causing unreasonable pain

GenFacades not supported assembly fails for reference source files that use the `=> throw null;` syntax instead of a full method body.

```
C:\git\runtime5\artifacts\obj\System.Net.Quic\net6.0-Browser-Debug\System.Net.Quic.notsupported.cs(17,9): error CS8057: Block bodies and expression bodies cannot both be provided. [C:\git\runtime5\src\libraries\System.Net.Quic\src\System.Net.Quic.csproj]

public QuicListener(IPEndPoint listenEndPoint, System.Net.Security.SslServerAuthenticationOptions sslServerAuthenticationOptions) { throw new System.PlatformNotSupportedException(System.SR.SystemNetQuic_PlatformNotSupported); }
=> throw null;
```

cc @Anipik @ericstj