Fix nullability issues in WrappingDirectoryImpl

Author: ascopes

java-compiler-testing/src/main/java/io/github/ascopes/jct/workspaces/impl/WrappingDirectoryImpl.java

@@ -101,6 +101,7 @@ public byte[] asJar() {
     });
   }
 
+  @Nullable
   @Override
   public PathRoot getParent() {
     return parent;