Computed entity reference fields break JsonApiGenerator

Problem/Motivation

If you have an entity type with a computed entity reference base field, the JsonApiGenerator class's getPaths() method breaks because it tries to call getCardinality() on NULL. This is because EntityFieldManagerInterface::getFieldStorageDefinitions() specifically ignores computed base fields.

Proposed resolution

Use getFieldDefinitions() instead, which does not ignore computed base fields.

Remaining tasks

Fix it, plus a regression test.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

None.