Ensure proper handling of Nullable Strings in C API

[CryZe]

Seems like we don't properly handle this in most of the language bindings.

Languages to check / fix:

Output:

• [x] ~~JavaScript / TypeScript for emscripten~~
• [x] JavaScript / TypeScript for Node
• [x] JavaScript / TypeScript for WASM
• [x] C#
• [ ] Java JNA
• [ ] Java JNI
• [ ] Kotlin JNI
• [ ] Swift
• [ ] Ruby
• [x] Python

Input (apparently we have no function that takes a nullable string as input atm):

• [x] ~~JavaScript / TypeScript for emscripten~~
• [ ] JavaScript / TypeScript for Node
• [ ] JavaScript / TypeScript for WASM
• [ ] C#
• [ ] Java JNA
• [ ] Java JNI
• [ ] Kotlin JNI
• [ ] Swift
• [ ] Ruby
• [ ] Python