Fix a bug with empty factors

[jojonas]

I encountered a bug where the web server would reply with an empty body on the probe request, but with a body if a certain parameter was specified. Arjun failed to recognize this, because the probe-phase set factors['same_body'] to an empty string.

During comparison, this would then be evaluated to False and the body-comparison would be skipped.

This commit fixes this bug by using the explicit None value as the "skip" placeholder.