fix: normalize_process_exception() test

[Chris53897]

i am not sure why $result['error_output'] is emtpy now in the ci runs, and was not before.
Changes in symfony?

[kbond]

It only fails in the code coverage job? That seems weird...

[Chris53897]

It fails on different runs. I see some action runs, that is was on code coverage. But there are others if i remember correct.

I will try to find some time, to have a deeper look.

Now it is NULL instead of '' ?

1) Zenstruck\Messenger\Monitor\Tests\History\ResultNormalizerTest::normalize_process_exception
Failed asserting that null is identical to 127.

[kbond]

I've hopefully made this test less brittle in #101