When creating a new subfield for a framework, the subfield will silently not save if the "Text for librarian" field isn't populated.

Reproduce issue:

1. Go to Administration->MARC bibliographic framework
2. Edit the Default framework by clicking the "Action" button and selecting "Marc Framework"
3. Search for tag 500
4. Click on the "Action" button and select "Edit subfields"
5. Click on the "New" tab
6. Enter 9 into the subfield code and leave the rest default
7. Hit "Save changes" button

RESULT:
List of all the subfields is shown with no subfield 9

Expected:
List to include subfield 9

Repeating the same processes but adding "Some Text" to the "Text for librarian" field results in a proper save.