Add license: pressing Save button leads to a 500 error

Description

In the Add License screen (`/admin/license/new'), if you click the Upload license button without indicating a file, this leads to an error 500.

Specificaiton

• We shouldn't be able to submit the form if no data is entered.

@jschatz1 I think we can solve this frontend wise, or should we do it in the backend?

@regisF This is a duplicate of https://gitlab.com/gitlab-org/gitlab-ee/issues/931

Status changed to closed

Thanks @grzesiek

@regisF I also added GIF prepared by you to mentioned issue that also contains a stack trace of error. Thanks for preparing it