CE output in non-default view mode is missing fields if no CE display present

@fago you had reported this weeks ago, but I was unable to reproduce it at that moment. (Likely because I only checked sites where a default CE display was already saved.)

The fix is straightforward. It just took me a while to work out

• that this is IMHO the appropriate location to add it;
• why the tests did not notice things and how they should be fixed. (Answer: for all auto-created / auto-processing CE displays, I should check which entity view displays the fields come from. Now done, and I trust the tests. The combination of permutations is... daunting, but I kept things "as readable as I can". Just don't read them, though.)

Related: 📌 Fix confusion in UI when no CE display exists Active gives info / warning message about what is happening when no CE displays exist for a bundle.

thx! works great and even has solid test coverage! let's merge!