fix(lsp): handle non-string documentation in completion items #38291

Problem: `get_doc` throws error with "attempt to get length of a userdata
value" when `item.documentation` is truthy but not a string (e.g. vim.NIL
from a JSON null).

Solution: Check `type(item.documentation)` before taking its length.

runtime/lua/vim/lsp/completion.lua

@@ -261,7 +261,7 @@ local function get_doc(item)
   if
     has_completeopt('popup')
     and item.insertTextFormat == protocol.InsertTextFormat.Snippet
-    and #(item.documentation or '') == 0
+    and (type(item.documentation) ~= 'string' or #item.documentation == 0)
     and vim.bo.filetype ~= ''
     and (item.textEdit or (item.insertText and item.insertText ~= ''))
   then