hie sends invalid message on hover

[Hogeyama]

When there is no information available against a hover request, hie returns a response like

{"result":{"contents":null},"jsonrpc":"2.0","id":1}

which does not respect the Language Server Protocol in which contents cannot be null:

interface Hover {
    contents: MarkedString | MarkedString[] | MarkupContent;
    range?: Range;
}

This PR includes a temporal measure for this problem.
It would be better to fix haskell-lsp so as not to allow null in HoverContents.
I've opened the issue on haskell-lsp too.

[alanz]

See discussion at haskell/lsp#162