Improve label handling on usage page

Problem/Motivation

In the usage page we show labels of entities that reference other entities.
For this we are special-casing paragraphs because they normally indicate their parent's labels. In some circumstances, paragraphs can return empty labels, and when we encounter that, we are just skipping the row altogether, instead of showing a row with an empty label. ( #2954039: Fix ::label() logic and add a ParagraphInterface::getOrphanStatus() helper → will likely solve the "empty" label in paragraphs, at some point)

We could see how to better generalize this situation. Maybe other entities referencing target entities could also return an empty label, and would also need some sort of fallback.

Proposed resolution

Investigate how we could have a better fallback for source entities that don't have a label, when showing usages on the list usage page.

Remaining tasks

User interface changes

API changes

Data model changes