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No, nullable fields in the builder are expected.
By default, when a builder instance is created, all values are set to their defaults (null for reference fields) - thus, it's necessary to mark these fields @Nullable.
This initial Builder state represents an incomplete configuration, and the EnumValue should not be constructible in this state - calling the build() method should result in an IllegalStateException.

I planned to add something like this later:

public EnumValue build() {
    if (name == null) {
        throw new IllegalStateException("name must be set");
    }
    return new EnumValue(name, sourceLocation, comments, ignoredChars, additionalData);
}