Sections cannot be emptied

Sections are never emptied.

Steps to reproduce:
1. enter text in all sections
2. save and view the page
3. remove the text in two sections
4. save and view the page
5. you will see that the sections that you removed are still rendered in the page, but without any contents. But when you have styling applied, for example borders, you will see these on the page.

This is solved by adding a "Empty" button with the sections.
The patch is in field_collection:
#1413864: Provide an empty button when field cardinality is different than unlimited