Make multi-network construction work with igraph version 2.0.1.1

When adding an empty set of artifact vertices to the multi-network,
adding 'no' vertex fails as of igraph 2.0.1.1. This is easily
fixed by checking whether the set of arftiact vertices to add is
empty, and then just don't call the add function if it is empty.

Signed-off-by: Thomas Bock <bockthom@cs.uni-saarland.de>

util-networks.R

@@ -958,8 +958,10 @@ NetworkBuilder = R6::R6Class("NetworkBuilder",
             ## to be consistent with bipartite networks
             artifacts.to.add.kind[artifacts.to.add.kind == "IssueEvent"] = "Issue"
 
-            artifacts.net = artifacts.net + igraph::vertices(artifacts.to.add, type = TYPE.ARTIFACT,
-                                                             kind = artifacts.to.add.kind)
+            if (length(artifacts.to.add) > 0) {
+                artifacts.net = artifacts.net + igraph::vertices(artifacts.to.add, type = TYPE.ARTIFACT,
+                                                                 kind = artifacts.to.add.kind)
+            }
 
             ## check directedness and adapt artifact network if needed
             if (igraph::is.directed(authors.net) && !igraph::is.directed(artifacts.net)) {