null value fields in payload should be skipped

[karthiknadig]

I am not sure if the LSP spec specifies if you can use null for optional fields in response or notification. But this seems to cause issues with VS Code.

Here is an example, consider this response content from pygls:

[
    {
        "name": "SampleClass",
        "kind": 5,
        "range": {
            "start": {
                "line": 17,
                "character": 0
            },
            "end": {
                "line": 19,
                "character": 12
            }
        },
        "selectionRange": {
            "start": {
                "line": 17,
                "character": 6
            },
            "end": {
                "line": 17,
                "character": 17
            }
        },
        "detail": null,
        "children": [
            {
                "name": "__init__",
                "kind": 6,
                "range": {
                    "start": {
                        "line": 18,
                        "character": 4
                    },
                    "end": {
                        "line": 19,
                        "character": 12
                    }
                },
                "selectionRange": {
                    "start": {
                        "line": 18,
                        "character": 8
                    },
                    "end": {
                        "line": 18,
                        "character": 16
                    }
                },
                "detail": null,
                "children": [],
                "deprecated": false
            }
        ],
        "deprecated": false
    }
]

Notice the "detail": null, looks like VS Code does not like that. Outline does not show up in VS Code UI when "detail": null.

Removing that null fields seems fix the issue with Outline not showing. This is the same case with CodeActions as well where the optional fields get null and it does not show up in VS Code UI.

Typical CodeAction response:

```json
{
        "title": "Extract expression into function 'func_njmjvxyh'",
        "kind": "refactor.extract",
        "diagnostics": null,
        "edit": {
            "changes": null,
            "documentChanges": [
                {
                    "textDocument": {
                        "uri": "file:///c:/GIT/repro/lsptest/tests/test_math.py",
                        "version": null
                    },
                    "edits": [
                        {
                            "range": {
                                "start": {
                                    "line": 18,
                                    "character": 8
                                },
                                "end": {
                                    "line": 18,
                                    "character": 8
                                }
                            },
                            "newText": "func_njmjvxyh(self):\n        \r\n        return \n\n    def "
                        },
                        {
                            "range": {
                                "start": {
                                    "line": 19,
                                    "character": 12
                                },
                                "end": {
                                    "line": 19,
                                    "character": 14
                                }
                            },
                            "newText": " = self.func_njmjvxyh()\n"
                        }
                    ]
                }
            ]
        },
        "command": null
    }

Related: https://github.com/microsoft/vscode/issues/115793
Related: pappasam/jedi-language-server#60

[krassowski]

Already tracked in #124 I believe. The consensus appears to be that nulls are not allowed unless explicitly specified in the protocol.

[danixeee]

@karthiknadig

As @krassowski said, we are aware of that and the bug should be fixed in #139. It will be merged and released soon, but there will be backwards-incompatible changes.

[danixeee]

Solved in #139.