TMSS-190: check for key 'time.at'

SAS/TMSS/services/scheduling/lib/constraints/template_constraints_v1.py

@@ -140,7 +140,7 @@ def get_earliest_possible_start_time(scheduling_unit: models.SchedulingUnitBluep
     constraints = scheduling_unit.draft.scheduling_constraints_doc
 
     try:
-        if has_manual_scheduler_constraint(scheduling_unit):
+        if has_manual_scheduler_constraint(scheduling_unit) and 'at' in constraints['time']:
             at = parser.parse(constraints['time']['at'], ignoretz=True)
             return at