Abnormal behavior in issue api response

[AutomationUIUser]

The api response for sprint gives following for Jira on premise verison v8.13.1

com.atlassian.greenhopper.service.sprint.Sprint@30fa5ecc[id=5542,rapidViewId=1197,state=ACTIVE,name=EB-SSW PI21-05-4_26.04.,startDate=2021-04-13T17:00:00.000+02:00,endDate=2021-04-26T17:07:00.000+02:00,completeDate=<null>,activatedDate=2021-04-13T17:01:22.902+02:00,sequence=5071,goal=,autoStartStop=false]

whereas on another instance of same version we get response like  

"com.atlassian.greenhopper.service.sprint.Sprint@f54ecbe[id=29107,rapidViewId=350,state=FUTURE,name=TESR4J Sprint 1,startDate=<null>,endDate=<null>,completeDate=<null>,activatedDate=<null>,sequence=29107,goal=<null>,autoStartStop=false]"

In both of these if we check ;goal' field, in first instance when goal is empty we get 'goal='  whereas in second instance when goal is empty we get goal=<null>.

Could I possibly know the reason of this behaviour.