Please use following JSON codes as example:
"elmType": "div",
"txtContent": "=if ([$ChangeContact] == '', 'Change Manager', 'Change Manager: ' + join([$ChangeContact.title], ', ')"
"attributes": {
"class": "ms-fontColor-white ms-fontWeight-bold ms-fontSize-12"
Result:
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.