Handle Empty CodeActionKind

[tsukkee]

This PR makes sorbet-lsp supports Empty CodeActionKind.
spec: https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#codeActionKind

For example, vim-lsp sends Empty CodeActionKind in only property of CodeActionContext.
code: https://github.com/prabirshrestha/vim-lsp/blob/3af8f3b38effc4a631a15bb283a4b701c251275d/autoload/lsp/ui/vim/code_action.vim#L55

Related slack thread: https://sorbet-ruby.slack.com/archives/CFT8Y4909/p1701997597443059

Motivation

Vim with vim-lsp users cannot use Sorbet's CodeAction because sorbet-lsp returns the following error message when vim-lsp requests CodeAction.

Unable to deserialize LSP request: Error deserializing JSON message: Invalid value for enum type `CodeActionKind`: 

This error message looks like truncated because the error is caused by an empty string (""), which represents Empty CodeActionKind.

Test plan

I am sorry but I don't have good idea to write code to check make_lsp_types.cc follows the spec.
I would like to have some advice to do that if I need to write test in this case.