cubicweb #2522284 wrap_on_write does wrap when writing the empty string [open]

This can be considered a bug or not, but should at least be documented (and tested would be even better).

I would say it is a bug considering the intented usage, otherwise we should fix RsetBox.render_title and similar methods, which write an empty string when using uicfg showlabel=False method to make sidebox titles disappear.

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