Epic Name can be saved as empty when changing issue type

Epic Name is declared as mandatory when creating/editing or moving a issue, but it's possible the field is stored null/empty

Steps to reproduce:

1. Create a non epic issue
2. Change issue type to Epic (not moving the issue but selecting the issue type and selecting Epic from the options)

• Changed issue will now have an empty Epic text value
• Trying to edit/save the new issue will trigger the proper validation, but the value in the database will still be null
• Software board show the text as in unlabelled-ISSUE-KEY