fix(progress): treat vim.NIL as nil in LSP progress fields

[1334]

Some LSP servers emit $/progress notifications whose optional value.title, value.message, or value.percentage fields arrive as JSON null, which vim.lsp decodes as vim.NIL. vim.NIL is truthy in Lua but isn't a string/number, so passing it through M.poll_for_messages() unchanged crashes downstream display code on string concat / formatting.

This coerces vim.NIL → nil for those three fields before constructing the message table. Non-nil, non-vim.NIL values pass through unchanged.

Minimal change, scoped to lua/fidget/progress/lsp.lua. I've been running this locally for a while without regressions against servers that don't send null (they hit the ~= vim.NIL short-circuit untouched).

[j-hui]

Thank you fixing this!