backport: fix(lsp): util.lua attempt to concatenate userdata #39510

Problem:
Error when querying document symbols using python-lsp-server:

    lsp/util.lua:1955: attempt to concatenate field 'containerName' (a userdata value)

Solution:
Check for `vim.NIL`.

(cherry picked from commit 1799aaebda)

runtime/lua/vim/lsp/completion.lua

@@ -36,6 +36,7 @@ local M = {}
 local api = vim.api
 local lsp = vim.lsp
 local protocol = lsp.protocol
+local isnil = require('vim._core.util').isnil
 
 local rtt_ms = 50.0
 local ns_to_ms = 0.000001
@@ -1009,7 +1010,7 @@ local function trigger(bufnr, clients, ctx)
             client and client.name or 'UNKNOWN'
           )
         )
-      elseif result and result ~= vim.NIL and #(result.items or result) > 0 then
+      elseif not isnil(result) 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