added notification to empty book notes otherwise they look suspiciously empty

src/services/task_context.js

@@ -14,6 +14,12 @@ class TaskContext {
         // progressCount is meant to represent just some progress - to indicate the task is not stuck
         this.progressCount = 0;
         this.lastSentCountTs = Date.now();
+
+        // just the fact this has been initialized is a progress which should be sent to clients
+        // this is esp. important when importing big files/images which take long time to upload/process
+        // which means that first "real" increaseProgressCount() will be called quite late and user is without
+        // feedback until then
+        this.increaseProgressCount();
     }
 
     /** @return {TaskContext} */