Runtime context is not initialized as a default

[eric-burel]

runtime.context is now a NoneType as a default, so this template only works with langgraph==0.6.0 and no other version. Running the agent with the latest version 0.6.0 will lead to an error when accessible runtime.context.get("changeme").

I am not sure yet how to instantiate a default context but I'll open a PR as soon as I find out to complete #13