sqldb+graph/db: fix UpsertNode bug

Account for the last_update field being null.

graph/db/graph_test.go

@@ -3361,18 +3361,7 @@ func TestAddChannelEdgeShellNodes(t *testing.T) {
 	require.ErrorIs(t, err, ErrEdgeAlreadyExist)
 
 	// Show that updating the shell node to a full node record works.
-	err = graph.AddLightningNode(node2)
-	_, ok := graph.V1Store.(*KVStore)
-	if ok {
-		require.NoError(t, err)
-	} else {
-		// Currently the SQL UpsertNode query prevents us from updating
-		// the node record if the current record's last_update field
-		// is null (which is the case for node2). This is a bug that
-		// will be fixed in the following commit.
-		require.ErrorContains(t, graph.AddLightningNode(node2),
-			"sql: no rows in result set")
-	}
+	require.NoError(t, graph.AddLightningNode(node2))
 }
 
 // TestNodePruningUpdateIndexDeletion tests that once a node has been removed