Collection edit has empty entity type dropdown if no entities / relationship types initialised

[MarieVerdonck]

Describe the bug
When you don't have a DB with the entity types initialised (with script initialize-entities -f ../dspace/config/entities/relationship-types.xml) the 'Entity Type' dropdown in collection is empty, causing some confusion.
If the repository does not work with entity types & relationships then they don't need to be initialised but then also this dropdown should not be present in collection create / edit form.

To Reproduce
Steps to reproduce the behavior:

1. Start with empty DB or DB with no entity types initialised
2. Go to create and/or edit collection form
3. The entity type input (dropdown) exists but is empty

Expected behavior
When there are no entity types, then this field should not exist on the create and edit collection form.

[tdonohue]

Pulling this over to the 7.6.x maintenance board as we should try to fix this in a 7.6.x bug-fix release