fix NullPointerException in ClassesToRetransformFinder (#10316)

add check against null from classes returned by
Instrumentation::getAllLoadedClasses

Author: jpbempel

dd-java-agent/agent-debugger/src/main/java/com/datadog/debugger/agent/ClassesToRetransformFinder.java

@@ -43,6 +43,11 @@ public List<Class<?>> getAllLoadedChangedClasses(
     List<Class<?>> classesToBeTransformed = new ArrayList<>();
     Trie changedClasses = getAllChangedClasses(comparer);
     for (Class<?> clazz : allLoadedClasses) {
+      if (clazz == null) {
+        // clazz could be null in the array returned by Instrumentation::getAllLoadedClasses
+        // probably because class unloading
+        continue;
+      }
       if (lookupClass(changedClasses, clazz)) {
         classesToBeTransformed.add(clazz);
       }