Return a "false" schema for creation of workflow transition when there's no choice

Previously, we no transition was possible to fire, we would return an
empty array in the "oneOf" of "name" property, which is invalid (see
https://tools.ietf.org/html/draft-handrews-json-schema-validation-01#section-6.7.3).

So we return a "false" Boolean schema in these cases, thus clearly
indicating that no transition can be fired.

cubicweb_jsonschema/mappers/entity.py

@@ -314,6 +314,8 @@ class TrInfoEntityMapper(ETypeMapper):
         _ = self._cw._
         transitions_choice = [{'enum': [tr.name], 'title': _(tr.name)}
                               for tr in wfobj.possible_transitions()]
+        if not transitions_choice:
+            return False, None
         schema = {
             'type': 'object',
             'properties': {

test/test_mappers.py

@@ -1669,6 +1669,23 @@ class WfobjsMapperTC(CubicWebTC):
         }
         self.assertEqual(schema, expected)
 
+    def test_trinfo_creation_schema_empty(self):
+        with self.admin_access.cnx() as cnx:
+            user = self.create_user(cnx, u'bob')
+            cnx.commit()
+            mapper = self.vreg['mappers'].select(
+                'jsonschema.entity', cnx, etype='TrInfo',
+                for_entity=user)
+            # Take advantage of adapters being cached to patch the object
+            # directly.
+            wfobj = user.cw_adapt_to('IWorkflowable')
+            with patch.object(wfobj, 'possible_transitions',
+                              return_value=[]) as patched:
+                schema = mapper.json_schema('creation')
+            patched.assert_called_once()
+        expected = False
+        self.assertEqual(schema, expected)
+
     def test_collection_serialize(self):
         with self.admin_access.cnx() as cnx:
             user = self.create_user(cnx, u'bob')