Use empty string instead of NULL as default signature doc

[renkun-ken]

With latest vscode-languageclient 7.0.0 used in vscode-r-lsp 0.1.14, the following code does not trigger SignatureHelp:

fun <- function(x, y) {
  x + y
}

fun(

It looks likedocumentation === null is no longer accepted by vscode-languageclient 7.0.0. The LSP says the following

documentation?: string | MarkupContent;

looks like documentation could not be null.

This PR is a simple fix and it works with latest vscode-r-lsp now.

[randy3k]

LGTM