[BUG] Undefined array key 3 warning when using date interval in JSON-LD

**What steps does it take to reproduce the issue?**

Create an item with and EDTF interval, e.g. '2022/2023', and view the JSON-LD. This creates a warning in the logs stating "Undefined array key 3" because the converter is _expecting a single date_.

**What did you expect to happen?**

No warnings. It should return the earliest ISO 8601 date value for the interval.
