fix empty sprints causing an exception to be thrown

usecases/sprints/result.go

@@ -17,10 +17,10 @@ type Result struct {
 
 	PartIDs []int `json:"partIds"` // Just for simplifying code.
 
-	Items             []items.Result               `json:"items,omitempty"`
-	Stats             []entities.Stat              `json:"stats,omitempty"`
-	Requirements      []projects.RequirementResult `json:"requirements,omitempty"`
-	Progress          []ResultProgress             `json:"progress,omitempty"`
+	Items             []items.Result               `json:"items"`
+	Stats             []entities.Stat              `json:"stats"`
+	Requirements      []projects.RequirementResult `json:"requirements"`
+	Progress          []ResultProgress             `json:"progress"`
 	ItemBurndown      []BurndownDataPoint          `json:"itemBurndown,omitempty"`
 	AggregateBurndown []BurndownDataPoint          `json:"aggregateBurndown,omitempty"`
 }