This can happen if emailAddress is missing from JIRA API response.
As per JIRA docs:
If using the get issue REST API endpoint with Jira you find that the reporter account is not returning an email address, it is likely the user has configured their profile settings to hide their email address.
This can happen if emailAddress is missing from JIRA API response.
As per JIRA docs:
https://confluence.atlassian.com/jirakb/how-to-display-the-email-address-of-the-reporter-on-a-jira-issue-1188416840.html