fix: handle willRenameFiles result.result nil

[Mrreadiness]

Now will_do can send result.result=nil to vim.lsp.util.apply_workspace_edit which expect not nil WorkspaceEdit. In some cases (for example when a rust analyzer is enabled) it leads to error.
Be lsp spec willRenameFiles can return an empty result.result. So I added handling of this case. Also, it doesn't look like an error, so there is no error message.

[jamestrew]

Thanks!