When Bitbucket communicates with Jira about one or several commits that are linked to an issue, Bitbucket can sometimes send the field initiator:email as 'null'. This behaviour can cause an unexpected error in Jira as it always expects a value for that field.
This issue has only been reproduced on repositories that use the SVN Mirror for Bitbucket Server app. For some reason, the commits can be created with missing fields and then Bitbucket sends those commits to Jira with missing data.
Synthetic reproduction: This issue can be reproduced by manually setting "email" field into null in cwd_user table. The actual reason why "SVN Mirror for Bitbucket Server" causes Bitbucket to report commits in the way it does is still unknown.
When there is a situation where a commit does not contain an email address linked to it, Bitbucket should send an empty collection instead.
Bitbucket is sending the initiator email field as collection with single 'null' element.