fix(lsp): error on omnifunc completion (#37790)

Problem:
After eaacdc9, complete with emmylua_ls error with:
runtime/lua/vim/lsp/completion.lua:586: attempt to get length of field
'items' (a nil value)

Solution:
Result can be CompletionItem[] according the spec:
> If a `CompletionItem[]` is provided, it is interpreted to be complete,
> so it is the same as `{ isIncomplete: false, items }`

runtime/lua/vim/lsp/completion.lua

@@ -583,7 +583,7 @@ local function trigger(bufnr, clients, ctx)
       end
 
       local result = response.result
-      if result and #result.items > 0 then
+      if 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