TaskWriter must not write nonexistent target documents

Theoretically, when a document is deleted, its tasks get deleted by the DeleteTaskForDeletedDocumentListener.
Yet some cases were noticed where a task targeting a deleted document remains.

In such a case, we don't want the TaskWriter to write the document id in the "targetDocumentIds" field because the client won't know how to handle it.
The issue was seen on https://webui-demo.nuxeo.com/ on a task targeting a nonexistent document making the mobile app fail when fetching the user's tasks.

Faulty code in TaskWriter:

for (String docId : item.getTargetDocumentsIds()) {
    if (isFetchTargetDocumentIds) {
        IdRef idRef = new IdRef(docId);
        if (wrapper.getSession().exists(idRef)) {
            writeEntity(wrapper.getSession().getDocument(idRef), jg);
            break;
        }
    }
    jg.writeStartObject();
    jg.writeStringField("id", docId);
    jg.writeEndObject();
}
jg.writeEndArray();