fix: language server diagnostics return empty list instead of null

[mitsaucepls]

Hello, I noticed that I get an error when I try to edit an empty openapi.yaml file.

Specificly:

Error executing vim.schedule lua callback: /usr/share/nvim/runtime/lua/vim/lsp/diagnostic.lua:214: attempt to get length of local 'diagnostics' (a nil value)

stack traceback:
        /usr/share/nvim/runtime/lua/vim/lsp/diagnostic.lua:214: in function 'handle_diagnostics'
        /usr/share/nvim/runtime/lua/vim/lsp/diagnostic.lua:240: in function 'handler'
        /usr/share/nvim/runtime/lua/vim/lsp/client.lua:1117: in function ''
        vim/_editor.lua: in function <vim/_editor.lua:0>

This resulted from returning nil instead of an empty list as diagnostics.
As required by the specification: https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#publishDiagnosticsParams

I also renamed interface{} to any. Since its the new style.

[daveshanley]

LGTM