Return empty graph if no update report is found

src/main/scala/net/virtualvoid/sbt/graph/DependencyGraphSettings.scala

@@ -122,7 +122,7 @@ object DependencyGraphSettings {
   def moduleGraphIvyReportTask = ivyReport map (absoluteReportPath.andThen(IvyReport.fromReportFile))
   def moduleGraphSbtTask =
     (ignoreMissingUpdate, crossProjectId, configuration) map { (update, root, config) ⇒
-      SbtUpdateReport.fromConfigurationReport(update.configuration(config.name).get, root)
+      update.configuration(config.name).map(report ⇒ SbtUpdateReport.fromConfigurationReport(report, root)).getOrElse(ModuleGraph.empty)
     }
 
   def printAsciiGraphTask =

src/main/scala/net/virtualvoid/sbt/graph/model.scala

@@ -36,6 +36,10 @@ case class Module(id: ModuleId,
   def isEvicted: Boolean = evictedByVersion.isDefined
 }
 
+object ModuleGraph {
+  val empty = ModuleGraph(Seq.empty, Seq.empty)
+}
+
 case class ModuleGraph(nodes: Seq[Module], edges: Seq[Edge]) {
   lazy val modules: Map[ModuleId, Module] =
     nodes.map(n ⇒ (n.id, n)).toMap