backport: fix(lsp): show meaningful error on invalid completion response (#39476)

Problem: vim.NIL is truthy in Lua, so `#(result.items or result)`
crashes on `#vim.NIL` when servers return null.

Solution: skip spec-allowed result=null silently, raise an error
on items=null with the server name.

https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_completion
(cherry picked from commit b9431b340f)

Co-authored-by: glepnir <glephunter@gmail.com>

runtime/lua/vim/lsp/completion.lua

@@ -1003,7 +1003,13 @@ local function trigger(bufnr, clients, ctx)
       end
 
       local result = response.result
-      if result and #(result.items or result) > 0 then
+      if type(result) == 'table' and result.items == vim.NIL then
+        error(
+          ('%s: completion response has items=null, expected CompletionItem[]'):format(
+            client and client.name or 'UNKNOWN'
+          )
+        )
+      elseif result and result ~= vim.NIL and #(result.items or result) > 0 then
         Context.isIncomplete = Context.isIncomplete or result.isIncomplete
         local encoding = client and client.offset_encoding or 'utf-16'
         local client_matches, tmp_server_start_boundary