[FLINK-15739] Fix TypeSerializerUpgradeTestBase on Java 12

The problem was that some of the relocated classes in
PojoSerializerUpgradeTest have enums in them. Java 12 introduced the
EnumDesc class and enums will have an inner subclass of this defined
that does not have a ClassLoader. We now filter out classes that don't
have a classloader in the ClassRelocator because these classes don't
have bytecode that we could relocate.

flink-core/src/test/java/org/apache/flink/api/common/typeutils/ClassRelocator.java

@@ -148,6 +148,7 @@ Map<String, byte[]> remap() {
 
  return renaming.getDefinedClassesUnderRoot()
  .stream()
+ .filter(klass -> klass.getClassLoader() != null)
  .collect(Collectors.toMap(renaming::newNameFor, classToTransform -> {
  ClassReader providerClassReader = classReaderFor(classToTransform);
  ClassWriter transformedProvider = remap(renames, providerClassReader);