libcamera: yaml-parser: Add additional tests

Add additional tests in preparation for upcoming modifications on the
yaml parser. These tests handle the case where the yaml file contains
empty items in dictionaries or lists. E.g.:

dict:
  key_with_value: value
  key_without_value:

Signed-off-by: Stefan Klug <stefan.klug@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@idea/* SPDX-License-Identifier: LGPL-2.1-or-later */
/*
 * Copyright (C) 2021, Ideas on Board Oy
 *
 * image.cpp - Multi-planar image with access to pixel data
#include <sys/mman.h>
#include <unistd.h>

using namespace libcamera;

std::unique_ptr<Image> Image::fromFrameBuffer(const FrameBuffer *buffer, MapMode mode)
{
	std::unique_ptr<Image> image{ new Image() };

	assert(!buffer->planes().empty());

	int mmapFlags = 0;

	if (mode & MapMode::ReadOnly)
		mmapFlags |= PROT_READ;

	if (mode & MapMode::WriteOnly)
		mmapFlags |= PROT_WRITE;

	struct MappedBufferInfo {
		uint8_t *address = nullptr;
		size_t mapLength = 0;
		size_t dmabufLength = 0;