[joern-export] Handle Empty CPG (joernio#3897)

Upon exporting `Representation.Cpg`, an iterator based on methods is started, and then the result is reduced.

If the iterator is empty, the reduce will throw an exception.

This change instead returns an empty result.

Resolves joernio#3892

joern-cli/src/main/scala/io/joern/joerncli/JoernExport.scala

@@ -1,15 +1,16 @@
 package io.joern.joerncli
 
-import better.files.Dsl._
+import better.files.Dsl.*
 import better.files.File
 import io.joern.dataflowengineoss.DefaultSemantics
-import io.joern.dataflowengineoss.layers.dataflows._
+import io.joern.dataflowengineoss.layers.dataflows.*
 import io.joern.dataflowengineoss.semanticsloader.Semantics
 import io.joern.joerncli.CpgBasedTool.exitIfInvalid
-import io.joern.x2cpg.layers._
+import io.joern.x2cpg.layers.*
 import io.shiftleft.codepropertygraph.Cpg
+import io.shiftleft.codepropertygraph.generated.NodeTypes
 import io.shiftleft.semanticcpg.language.{toAstNodeMethods, toNodeTypeStarters}
-import io.shiftleft.semanticcpg.layers._
+import io.shiftleft.semanticcpg.layers.*
 import overflowdb.formats.ExportResult
 import overflowdb.formats.dot.DotExporter
 import overflowdb.formats.graphml.GraphMLExporter
@@ -142,19 +143,23 @@ object JoernExport {
       case Representation.All =>
         exporter.runExport(cpg.graph, outDir)
       case Representation.Cpg =>
-        val windowsFilenameDeduplicationHelper = mutable.Set.empty[String]
-        splitByMethod(cpg).iterator
-          .map { case subGraph @ MethodSubGraph(methodName, methodFilename, nodes) =>
-            val relativeFilename = sanitizedFileName(
-              methodName,
-              methodFilename,
-              exporter.defaultFileExtension,
-              windowsFilenameDeduplicationHelper
-            )
-            val outFileName = outDir.resolve(relativeFilename)
-            exporter.runExport(nodes, subGraph.edges, outFileName)
-          }
-          .reduce(plus)
+        if (cpg.graph.nodeCount(NodeTypes.METHOD) > 0) {
+          val windowsFilenameDeduplicationHelper = mutable.Set.empty[String]
+          splitByMethod(cpg).iterator
+            .map { case subGraph @ MethodSubGraph(methodName, methodFilename, nodes) =>
+              val relativeFilename = sanitizedFileName(
+                methodName,
+                methodFilename,
+                exporter.defaultFileExtension,
+                windowsFilenameDeduplicationHelper
+              )
+              val outFileName = outDir.resolve(relativeFilename)
+              exporter.runExport(nodes, subGraph.edges, outFileName)
+            }
+            .reduce(plus)
+        } else {
+          emptyExportResult
+        }
       case other =>
         throw new NotImplementedError(s"repr=$repr not yet supported for this format")
     }
@@ -213,6 +218,8 @@ object JoernExport {
     )
   }
 
+  private def emptyExportResult = ExportResult(0, 0, Seq.empty, Option("Empty CPG"))
+
   case class MethodSubGraph(methodName: String, methodFilename: String, nodes: Set[Node]) {
     def edges: Set[Edge] = {
       for {