Fix exception when report message does not have begin

[kdarkhan]

I am not entirely sure about the root cause. I am using this plugin from lualine in order to get diagnostic counts.

I do use sumneko lua-language-server, and whenever I run PackerCompile, I believe lua LSP server is restarted, and messages are lost or something like that. I did not spend too much time investigating about the root cause. I just noted that adding this check similar to how it was done in https://github.com/nvim-lua/lsp-status.nvim/issues/6 resolves the issue.