A user is able to convert an issue to Epic without the need for Epic name.

An issue can be converted to an Epic without Epic name, under the following scenarios.
Issue Navigator
1. Bring the Edit dialog of an issue using the . dialog.
2. Change the issue type to Epic.
Rapid Board
1. Bring the Edit dialog of an issue using the . dialog.
2. Change the issue type to Epic.
View Issue Page
1. On the View issue page, inline edit the issue type to Epic.

Note that if the Edit button is click on the View Issue page and the issue type changed to Epic, then Epic Name field is marked as mandatory.