Protect null embeddings - v2.0 (#627)

* Don't emit graph embeddings if there aren't any.

* Don't store graph embeddings in a knowledge store if there's an empty list.

* Translate between Cassandra's 'null' representing an empty list and an
  empty list which is what the surrounding code wants (and stored in the
  first place).

* Avoid emitting empty embedding lists

* Avoid output empty triple lists

* Fix tests

trustgraph-flow/trustgraph/embeddings/graph_embeddings/embeddings.py

@@ -73,12 +73,13 @@ class Processor(FlowProcessor):
                     )
                 )
 
-            r = GraphEmbeddings(
-                metadata=v.metadata,
-                entities=entities,
-            )
+            if entities:
+                r = GraphEmbeddings(
+                    metadata=v.metadata,
+                    entities=entities,
+                )
 
-            await flow("output").send(r)
+                await flow("output").send(r)
 
         except Exception as e:
             logger.error("Exception occurred", exc_info=True)