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

@@ -1603,29 +1603,29 @@ 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(
-            this,
-            value,
-            descr,
-            collectionType.collection.elementtype,
-            parameterName,
-            isRawArray,
-          );
-        } else if (collectionType.collection.kind === spec.CollectionKind.Map) {
-          validateMap.call(
-            this,
-            value,
-            descr,
-            collectionType.collection.elementtype,
-            parameterName,
-          );
-        } else {
-          throw new Error(
-            `Unhandled collection kind: ${spec.describeTypeReference(type)}`,
-          );
+      } else if (spec.isCollectionTypeReference(type)) {
+        switch (type.collection.kind) {
+          case spec.CollectionKind.Array:
+            return validateArray.call(
+              this,
+              value,
+              descr,
+              type.collection.elementtype,
+              parameterName,
+              isRawArray,
+            );
+          case spec.CollectionKind.Map:
+            return validateMap.call(
+              this,
+              value,
+              descr,
+              type.collection.elementtype,
+              parameterName,
+            );
+          default:
+            throw new Error(
+              `Unhandled collection kind: ${spec.describeTypeReference(type)}`,
+            );
         }
       }
     }