cubicweb-medicalexp #3340518 Drop required constraint on Therapy's start_date [validation pending]

This should not be mandatory (sometimes, the date is unknown).

prioritynormal
typebug
done in0.6.0
load0.500
load left0.000
closed by#06b37eeaeb5c [schema] Drop required constraint on Therapy's start_date, closes #3340518
patch[schema] Drop required constraint on Therapy's start_date [applied]