Redirect output to /dev/null (#2880)

* Redirect output to devnull

Fixes #2877, #2878 and #2879.

* Rename file handle

* Reformat

Author: alexiri

src/textual/app.py

@@ -1978,8 +1978,10 @@ async def invoke_ready_callback() -> None:
                                 with redirect_stdout(redirector):  # type: ignore
                                     await run_process_messages()
                         else:
-                            with redirect_stderr(None):
-                                with redirect_stdout(None):
+                            with open(os.devnull, "w") as null_file:
+                                with redirect_stdout(null_file), redirect_stderr(
+                                    null_file
+                                ):
                                     await run_process_messages()
 
                 finally: