jmix-add-entity-event-listener: fix the isLoaded guard and cover a soft-deleted reference target

[glebfox]

Fixes #119.

Drafted from a field report while the problem was fresh, by the agent that hit it. It is unreviewed: treat the wording as a starting point, not a proposal to merge as-is.

Change

Two additions to "Reading a reference you nevertheless need":

• The guard in the code block becomes isLoaded(...) && value != null, with a paragraph explaining why — EntityStates.isLoaded reports every attribute of a DataManager.getReference instance as loaded, so the plain guard takes the wrong branch and the getter returns null.
• A paragraph on a soft-deletable target: the reload filters a soft-deleted row out and yields null indistinguishably from an unset reference, so the load needs PersistenceHints.SOFT_DELETION false.