Fix world importer breaking if there are no files to import

src/main/java/me/cortex/voxy/commonImpl/importers/WorldImporter.java

@@ -120,9 +120,13 @@ public class WorldImporter implements IDataImporter {
 
     @Override
     public void runImport(IUpdateCallback updateCallback, ICompletionCallback completionCallback) {
-        if (this.isRunning || this.worker == null) {
+        if (this.isRunning) {
             throw new IllegalStateException();
         }
+        if (this.worker == null) {//Can happen if no files
+            completionCallback.onCompletion(0);
+            return;
+        }
         this.isRunning = true;
         this.updateCallback = updateCallback;
         this.completionCallback = completionCallback;