Empty tooltips are displayed after a function call

Seems like we're returning something VSC can't handle.

[Trace - 10:26:59 AM] Received response 'textDocument/signatureHelp - (29)' in 1ms.
Result: {
    "signatures": [
        {
            "label": "foo()",
            "documentation": {
                "kind": "markdown",
                "value": ""
            },
            "parameters": []
        }
    ],
    "activeSignature": -1,
    "activeParameter": -1
}

Old LS does:

[Trace - 10:30:47 AM] Received response 'textDocument/signatureHelp - (1)' in 60ms.
Result: {
    "signatures": [
        {
            "label": "foo()",
            "documentation": {
                "kind": "plaintext",
                "value": ""
            },
            "parameters": [],
            "_returnTypes": [
                "int"
            ]
        }
    ],
    "activeSignature": 0,
    "activeParameter": -1
}