Journal customization function not consistent (minor)

[omangin]

Currently the journal customization returns a dict when the journal field is non empty (i.e. != '') and the empty string otherwise. This is not a really consistent behavior since a set of space in the journal field returns a dict. I believe it should either drop the field when it's space only or always return a dict whether the name is empty or not. The test from bibtexparser/customization.py#L420 is actually responsible for the behavior.