Conditional setting of certain fields in a builder

I have a legacy Java class with a builder - I can't change this class.

Some of the setters of the builder are throwing an exception on null input:

     public Cat.Builder setOwnerName(String value) {
        if (value == null) {
            throw new NullPointerException();
        } else {
            this.ownerName = value;
            this.onChanged();
            return this;
        }
     }

I'm trying to create that Cat from another class - using an external object that may, or may not, have a null value in the relevant field:

  Cat cat = Cat.newBuilder()
            .setXYZ("XYZ")
            .setOwnerName(inputFromUser.getOwnerName())
            .build();

And since the inputFromUser.getOwnerName() is sometimes null, the builder throws an exception.

I'm trying to find an elegant way to conditionally set or not the ownerName (or any other of the non-null fields).

Cat.Builder builder = Cat.newBuilder()
    .setXYZ("XYZ");
Optional.ofNullable(inputFromUser.getOwnerName())
    .ifPresent(builder::setOwnerName);
// repeat above statement for all optional fields
Cat cat = builder.build();

You don't have to use call chaining to invoke a builder:

Cat.Builder catBuilder = Cat.newBuilder();
catBuilder.setXYZ("XYZ");
catBuilder.setOwnerName(inputFromUser.getOwnerName());
Cat cat = catBuilder.build();

(You probably don't. There's no requirement for a Builder method to return itself, but it would be a little unusual if it didn't).

So, write it like this, and put a conditional where needed.

Cat.Builder catBuilder = Cat.newBuilder();
catBuilder.setXYZ("XYZ")
if (inputFromUser.getOwnerName() != null) {
  catBuilder.setOwnerName(inputFromUser.getOwnerName());
}
Cat cat = catBuilder.build();

If you wanted to be a bit more functional, you could write a method like:

<T> void consumeIfNonNull(@Nullable T value, Consumer<? super T> consumer) {
  if (value != null) consumer.accept(value);
}

and then do:

consumeIfNonNull(inputFromUser.getOwnerName(), catBuilder::setOwnerName);

I think this is neater at the call site than using an Optional, but to each their own.