cubicweb #1058267 Edition form crashes when mandatory relation is empty [resolved]

When a subject relation has a cardinality like '1x' (x being 1,? or *), and for some reason (incorrect data import for example) the relation was left empty, editing the subject entity in the web interface just crashes because complete_entity is called and crashes. It would be nice to be able to edit the entity with the web interface so that we can repair this situation manually.

ReST / HTML errors:System Message: WARNING/2 (&lt;string&gt; , line 1); <em>backlink</em></p>
Inline emphasis start-string without end-string.
done in3.9.0
load left0.000
closed by#7183f32fad13 [entity] fix .complete so we don't crash on missing mandatory relation. Also simplify code. Closes #1058267