[FIX] project_timesheet: don't trigger write if there is nothing to write

Triggering an empty write could break constraint (can not update past timesheet) (opw 606963)

addons/project_timesheet/project_timesheet.py

@@ -200,7 +200,8 @@ class project_work(osv.osv):
                 if amount_unit and 'amount' in amount_unit.get('value',{}):
                     vals_line['amount'] = amount_unit['value']['amount']
 
-            self.pool.get('hr.analytic.timesheet').write(cr, uid, [line_id.id], vals_line, context=context)
+            if vals_line:
+                self.pool.get('hr.analytic.timesheet').write(cr, uid, [line_id.id], vals_line, context=context)
 
         return super(project_work,self).write(cr, uid, ids, vals, context)