fix: guard against possible None sys.stdout

Operating System

Reproduces on windows

Esptool Version

4.8.1 and later

Python Version

3.11.x

Full Esptool Command Line that Was Run

N/A

Esptool Output

We are using esptool to flash firmware in the context of our app. 

This app runs on Kivy, and built with Pyinstaller. On windows, it appears that sys.stdout is not available (sys.stdout is None)

Under these conditions, esptool will crash when sys.stdout.flush() is called

What is the Expected Behaviour?

esptool should guard against this condition and not crash if sys.stdout is not available.

More Information

Reproduces on Windows with Kivy app built with pyinstaller

Other Steps to Reproduce

No response