Enable ignoring namespace guard for handle-marcxml in flux

[TobiasNx]

@blackwinter introduced the possibility to turn off the namespace guard with handle-marcxml but at the moment it only works in Java due to setting the option namespace=null. Assining null is not possible in FLUX.

         > So you mean that you can't use `namespace=""` with both records with and without namespace? That's where `namespace=null` comes in, but Flux does not support specifying `null` values.

Correct. We need to decide how to support this. Should I open a ticket to support this? Or is it a better idea to introduce an boolean-option that turns this test off?

Originally posted by @TobiasNx in #330 (comment)

hbz/Verbundgruppe has the usecase to use the same workflow for transformation marcxml WITH and WITHOUT namespace. This would require this enhancement.