DCOM-216: [GH-298] Silence E_NOTICE warning: "Undefined index".

Jira issue originally created by user @doctrinebot:

This issue is created automatically through a Github pull request on behalf of fredrik-w:

Url: https://github.com/doctrine/common/pull/298

Message:

Running PHP with error reporting on for E_NOTICE this bit me. Fix is simple and straightforward, check if key is defined and use that value if so, otherwise use null.
