Scalar handlers might return null

[io7m]

The following scalar handler:

public final class BTScalarElementHandler<S> implements BTElementHandlerType<Object, S>
{
  private final BTCharacterHandlerType<S> handler;
  private final BTQualifiedName name;
  private S result;

  /**
   * Construct a handler.
   *
   * @param inName    The name of elements handled by this handler
   * @param inHandler The character handler
   */

  public BTScalarElementHandler(
    final BTQualifiedName inName,
    final BTCharacterHandlerType<S> inHandler)
  {
    this.name = Objects.requireNonNull(inName, "name");
    this.handler = Objects.requireNonNull(inHandler, "handler");
  }

  @Override
  public String name()
  {
    return this.name.localName();
  }

  @Override
  public void onCharacters(
    final BTElementParsingContextType context,
    final char[] data,
    final int offset,
    final int length)
    throws Exception
  {
    this.result = this.handler.parse(context, data, offset, length);
  }

  @Override
  public S onElementFinished(
    final BTElementParsingContextType context)
  {
    return this.result;
  }
}

... Might return null. The reason: A closed element such as <element/> will cause the onCharacters method not to be called, resulting in this.result being unassigned.