Properly ignore stderr in elisp Flymake backend

Naively passing `null-device' as stderr creates a buffer named
"/dev/null" instead.  Pass a hidden buffer name instead.  (Bug#31902).

* lisp/progmodes/elisp-mode.el (elisp-flymake-byte-compile): Pass
hidden buffer as make-process :stderr instead of null-device.

lisp/progmodes/elisp-mode.el

@@ -1717,7 +1717,7 @@ current buffer state and calls REPORT-FN when done."
                            (format "byte-compile process %s died" proc))))
               (ignore-errors (delete-file temp-file))
               (kill-buffer output-buffer))))
-        :stderr null-device
+        :stderr " *stderr of elisp-flymake-byte-compile*"
         :noquery t)))))
 
 (defun elisp-flymake--batch-compile-for-flymake (&optional file)