Adding reviewers to a pull request can result an error if any users don't have an associated email

Issue Summary

Adding default reviewers to Pull requests can cause a "Database error occured" screen, if any user in Bitbucket dont have an associated email with their account.

Steps to Reproduce

1. Add some users in Bitbucket without email address associated with them(Sync from external LDAP)
2. Create a PR and add reviewers

Expected Results

Reviewers get added successfully.

Actual Results

Getting "An error occurred" pop-up.

The below exception is thrown in the atlassian-bitbucket.log file:

2021-11-24 11:36:21,515 ERROR [http-nio-7990-exec-5] bitbucket_user @1L2F8Q1x696x11892x0 fsvr8b 10.128.53.200 "GET /rest/ui/latest/projects/<proj_key>/repos/<repo_slug>/reviewer-groups HTTP/1.1" c.a.s.i.r.e.DefaultUnhandledExceptionMapperHelper Unhandled exception while processing REST request: "GET /rest/ui/latest/projects/<proj_key>/repos/<repo_slug>/reviewer-groups HTTP/1.1"
java.lang.NullPointerException: null 

Workaround

Associate an email address with each user account in Bitbucket.