Address PR feedback

Author: alextwoods

core/checksums/src/main/java/software/amazon/awssdk/checksums/internal/ConstructorCache.java

@@ -63,12 +63,17 @@ private Optional<Class<?>> getClass(String className) {
                 return Optional.empty();
             }
         });
+
         // if the WeakReference to the class has been garbage collected, remove it from the cache and try again
-        if (classRef.isPresent() && classRef.get().get() == null) {
+        if (classRef.isPresent()) {
+            Class<?> clazz = classRef.get().get();
+            if (clazz != null) {
+                return Optional.of(clazz);
+            }
             classesByClassLoader.remove(classLoader);
             return getClass(className);
         }
-        return classRef.map(WeakReference::get);
+        return Optional.empty();
     }
 
     /**