Failure generating HTML summary for xts-render profile results if certain tests fail and xtsttopng not available

Running the xts-render profile and trying to generate an HTML summary from the results can fail as below (if certain tests of the profile fail, and the xtsttopng program isn't available).

> ./piglit summary --overwrite html summary/xts-render results/xts-render.20201111-baseline                                                 ~/src/piglit-git/piglit
Traceback (most recent call last):
  File "./piglit", line 170, in <module>
    main()
  File "./piglit", line 166, in main
    sys.exit(runner(args))
  File "/home/daenzer/src/piglit-git/piglit/framework/exceptions.py", line 49, in _inner
    return func(*args, **kwargs)
  File "/home/daenzer/src/piglit-git/piglit/framework/programs/summary.py", line 115, in html
    summary.html(args.resultsFiles, args.summaryDir, args.exclude_details)
  File "/home/daenzer/src/piglit-git/piglit/framework/summary/html_.py", line 188, in html
    _make_testrun_info(results, destination, exclude)
  File "/home/daenzer/src/piglit-git/piglit/framework/summary/html_.py", line 122, in _make_testrun_info
    out.write(_TEMPLATES.get_template(
  File "/usr/lib/python3/dist-packages/mako/template.py", line 476, in render
    return runtime._render(self, self.callable_, args, data)
  File "/usr/lib/python3/dist-packages/mako/runtime.py", line 878, in _render
    _render_context(
  File "/usr/lib/python3/dist-packages/mako/runtime.py", line 920, in _render_context
    _exec_template(inherit, lclcontext, args=args, kwargs=kwargs)
  File "/usr/lib/python3/dist-packages/mako/runtime.py", line 947, in _exec_template
    callable_(context, *args, **kwargs)
  File "/tmp/daenzer/3198714166/python-3.8.6/mako-1.1.3/summary/html/html-summary/test_result.mako.py", line 46, in render_body
    __M_writer(str(image['image_ref']))
KeyError: 'image_ref'