Improve output for `NullByte` domain

[michael-schwarz]

There's two usability issues from #1076 that we didn't seem to notice:

• Currently, we always print the information from this domain, even when it is disabled, i.e., ana.base.arrays.nullbytes is false. This is confusing as it will always be top and users do not expect it to run at all.
• The XML output does not provide hints how to read results:

Ideally, the components should have speaking names such as MaySet and MustSet or something similar.