fix(lsp): unclear error on malformed server responses #40422

Problem:
Malformed server responses with `null` (vim.NIL) values, do not show
clear error messages.


Solution:
Assert two known, specific cases, to avoid user confusion.

runtime/lua/vim/diagnostic/_float.lua

@@ -211,6 +211,7 @@ function M.open(opts, ...)
 
     --- @type lsp.DiagnosticRelatedInformation[]
     local related_info = vim.tbl_get(diagnostic, 'user_data', 'lsp', 'relatedInformation') or {}
+    assert(related_info ~= vim.NIL, 'server response has invalid (null) relatedInformation')
     -- Below the diagnostic, show its LSP related information (if any) in the form of file name and
     -- range, plus description.
     for _, info in ipairs(related_info) do

runtime/lua/vim/lsp/diagnostic.lua

@@ -83,6 +83,7 @@ end
 --- @return table?
 local function tags_lsp_to_vim(diagnostic, client_id)
   local tags ---@type table?
+  assert(diagnostic.tags ~= vim.NIL, 'server response has invalid (null) tags')
   for _, tag in ipairs(diagnostic.tags or {}) do
     if tag == protocol.DiagnosticTag.Unnecessary then
       tags = tags or {}