Evaluate request returns null result if variable has None value.

[mfussenegger]

Environment data

• debugpy version: 1.6.2
• OS and version: Archlinux
• Python version (& distribution if applicable, e.g. Anaconda): 3.10
• Using VS Code or Visual Studio: Using nvim-dap

Actual behavior

Server returns:

{
  "seq": 20,
  "type": "response",
  "request_seq": 11,
  "success": true,
  "command": "evaluate",
  "body": {
    "result": null,
    "variablesReference": 0,
    "presentationHint": {}
  }
}

Expected behavior

{
  "seq": 20,
  "type": "response",
  "request_seq": 11,
  "success": true,
  "command": "evaluate",
  "body": {
    "result": "None",
    "variablesReference": 0,
    "presentationHint": {}
  }
}

In the specification the EvaluateResponse has result: string. (Not nullable, not optional).

Steps to reproduce:

Create a file with:

x = None
print(x)

Set a breakpoint at print(x) and trigger a evaluate request with expression = x and context = 'repl'.