Parameter converter triggers storage exception to be thrown

Problem/Motivation

1. Whenever SparqlEntityStorageConverter triest to convert an value with an entity type with a non-existing entity type ID, it causes an exception instead of returning NULL (so, users see a 5xx error instead of a 404).
2. Because the conversion is able to load entities with different storage than SparQL, it unnecessarily dispatches the SparqlEntityStorageEvents::GRAPH_ENTITY_CONVERT event.

Steps to reproduce

This was discovered with a path foo/{entity_type}/{entity} where the entity type is dynamic, and one tries to load the path /foo/missing_type/something. If this module is not installed, then visitors get a 404 response.

Expected: we get a 404 response (because of parameter is not converted)
Actual result: 500 response (PluginNotFoundException).

Proposed resolution

1. When the entity type is known, so ::applies() can be fully evaluated: check whether there is a definition for the given entity type ID before trying to load the storage. It makes no sense in the current order.
2. Do the same in ::convert when the entity type is dynamic, (so only ::applies is not invoked)

Remaining tasks

1. Test
2. Fix
3. Review

User interface changes

Nothing.

API changes

SparqlEntityStorageEvents::GRAPH_ENTITY_CONVERT is only fired if a SparQL-stored entity is converted.

Data model changes

Nothing.