[Task 71405] fixed the Process Files endpoints and added two tests for them

dspace-api/src/main/java/org/dspace/scripts/Process.java

@@ -8,6 +8,7 @@
 package org.dspace.scripts;
 
 import java.util.Date;
+import java.util.LinkedList;
 import java.util.List;
 import javax.persistence.Column;
 import javax.persistence.Entity;
@@ -176,6 +177,9 @@ public class Process implements ReloadableEntity<Integer> {
      * @return  The Bitstreams that are used or created by the process
      */
     public List<Bitstream> getBitstreams() {
+        if (bitstreams == null) {
+            bitstreams = new LinkedList<>();
+        }
         return bitstreams;
     }