fix issue when hoverData is nil

[njuCZ]

fix #317

tried to return empty lsp.Hover, however, the vscode client will complain: unsupported Markup content received. Kind is:

The official recommendation is just return null from here: microsoft/language-server-protocol#261

generally two ways：

• make lsp protocol model fields be pointer type.
• in handle function, just return pointer type result

It seems we generate the lsp protocol model, so I just choose the second way.
everything seems fine:

[radeksimko]

Good find and 👍 on the decision to use pointers here.

Thank you for the fix.

nil data is certainly a valid response and should be expected, in fact we should prefer that over the errors which are being returned for a number of edge cases, such as hovering over braces, but I haven't yet decided whether to just ignore them already in hcl-lang and never return such error in the first place or whether to filter out these errors and ignore them here.

[ghost]

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