fix(lsp): abort pending changes after flush when debouncing (#19314)

Issuing a server request triggers `changetracking.flush` so as to
make sure we're not operating on a stale state. This immediately
triggers notification of any pending changes (as a result of debouncing)
to the server. However, this happens in addition to the notification
that is waiting on the debounce delay. Because we `nil`
`buf_state.pending_change` when it is called, the fix is to
also check that this is non-`nil` when it is called and exit if it is,
as this being `nil` would mean that it originates from a pending change
that has already been flushed out.

runtime/lua/vim/lsp.lua

@@ -500,6 +500,9 @@ do
         table.insert(buf_state.pending_changes, incremental_changes(client, buf_state))
       end
       buf_state.pending_change = function()
+        if buf_state.pending_change == nil then
+          return
+        end
         buf_state.pending_change = nil
         buf_state.last_flush = uv.hrtime()
         if client.is_stopped() or not vim.api.nvim_buf_is_valid(bufnr) then