Bug: Default enum value for custom issue attribute does not apply

**Summary**

Currently it is possible to create a custom issue attribute with type enum, but the default value selected on the custom attribute does not apply when new issues are created.

**Steps to reproduce:**

1. In Settings > Classes > Issue, create an issue attribute with type enum.
2. Select a default enum value.
3. In the Tracker, create a new issue.

**Actual Result**

The custom attribute is visible in the right side panel, but does not reflect the default value.

**Expected Result**

The attribute should reflect the default value for the enum selected in Settings.