cubicweb #2236287 BFSS crash when updating value from None to None [validation pending]

In such situation the value is pop from cw_edited but never reinstalled.

priorityimportant
typebug
done in3.14.6
load0.500
load left0.000
closed by#87c72dccf7b9 [storage/bfss] ensure bfss never remove a cw_edited item (closes #2236287)