fix(lsp): handle nil root_dir in health check (#29010)

fix(lsp): handle nil root_dir in health check (#29007)

The root directory could show up as something like:

    Root directory: ~/path/to/cwd/v:null

Despite being `nil`

(cherry picked from commit f03b1622ad)

Co-authored-by: Mathias Fußenegger <mfussenegger@users.noreply.github.com>

runtime/lua/vim/lsp/health.lua

@@ -41,7 +41,10 @@ local function check_active_clients()
       end
       report_info(table.concat({
         string.format('%s (id: %d)', client.name, client.id),
-        string.format('  Root directory: %s', vim.fn.fnamemodify(client.root_dir, ':~')),
+        string.format(
+          '  Root directory: %s',
+          client.root_dir and vim.fn.fnamemodify(client.root_dir, ':~') or nil
+        ),
         string.format('  Command: %s', cmd),
         string.format('  Settings: %s', vim.inspect(client.settings, { newline = '\n  ' })),
         string.format(