Werk #7173: cmk-update-agent.exe: Fix stdout/stderr redirects

This Werk fixes the following bug:

cmk-update-agent.exe console output won't work on Windows if stdout/stderr gets redirected to file. Instead, a Traceback like this will be displayed for every output:

Traceback (most recent call last):
File "logging\__init__.py", line 861, in emit
File "logging\__init__.py", line 734, in format
File "cmk_update_agent.py", line 1713, in format
TypeError: encode() argument 1 must be string, not None
Logged from file cmk_update_agent.py, line 1009

The normal program flow is not affected by this bug, also normal logging takes place as expected, as can be seen at log/cmk-update-agent.log

To the list of all Werks