fix(pacmak): crash when generating java code

A missing condition in the code generator could have resulted
in attempting to read a property on undefined, resulting in
a crash.

packages/jsii-pacmak/lib/targets/java.ts

@@ -1603,26 +1603,26 @@ class JavaGenerator extends Generator {
  ) {
  if (spec.isUnionTypeReference(type)) {
  validateTypeUnion.call(this, value, descr, type, parameterName);
- } else {
- const collectionType = type as spec.CollectionTypeReference;
- if (collectionType.collection.kind === spec.CollectionKind.Array) {
- validateArray.call(
+ } else if (spec.isCollectionTypeReference(type)) {
+ switch (type.collection.kind) {
+ case spec.CollectionKind.Array:
+ return validateArray.call(
  this,
  value,
  descr,
- collectionType.collection.elementtype,
+ type.collection.elementtype,
  parameterName,
  isRawArray,
  );
- } else if (collectionType.collection.kind === spec.CollectionKind.Map) {
- validateMap.call(
+ case spec.CollectionKind.Map:
+ return validateMap.call(
  this,
  value,
  descr,
- collectionType.collection.elementtype,
+ type.collection.elementtype,
  parameterName,
  );
- } else {
+ default:
  throw new Error(
  `Unhandled collection kind: ${spec.describeTypeReference(type)}`,
  );