Return null when Hover request is cancelled or no symbol details

[ralish]

Previously we returned a new Hover instance without initializing the Contents or Range members. At least in VS Code, this will result in the LSP client attempting to access null properties. As we're not returning any Hover data, the correct response is to simply return null. See the Hover Request (Response) section in the LSP spec.

To reproduce the issue in VS Code:

1. Ensure the PowerShell extension is installed and open a PowerShell script.
2. Switch the Output console to Log (Extension Host) or Log (Window) (it seems to differ sometimes?).
3. Hover over some text with no expected hover content (e.g. a word in a string).
4. An exception is thrown on reading the uninitialized Contents property of the Hover instance.

[TylerLeonhardt]

LGTM thank you for fixing this!