Process an empty line in a block scalar.

def parse_block_scalar_empty_line(IndentTokenClass, ContentTokenClass):
    """Process an empty line in a block scalar."""
    def callback(lexer, match, context):
        text = match.group()
        if (context.block_scalar_indent is None or
                len(text) <= context.block_scalar_indent):
            if text:
                yield match.start(), IndentTokenClass, text
        else:
            indentation = text[:context.block_scalar_indent]
            content = text[context.block_scalar_indent:]
            yield match.start(), IndentTokenClass, indentation
            yield (match.start()+context.block_scalar_indent,
                    ContentTokenClass, content)
        context.pos = match.end()
    return callback