cubicweb-file #3064793 compute_sha1hex raise an exception if file.data is None [validation pending]

which may occur in a migration script

prioritynormal
typebug
done in1.14.1
load0.250
load left0.000
closed by#b81cbeb3b873 [entities] fix File.compute_sha1hex (closes #3064793)
patch[entities] fix File.compute_sha1hex (closes #3064793) [applied]