Undefined array key errors for responsive_image themeing without width or height

Problem/Motivation

#3359421: (Re-)Add width / height also on fallback image and #3192234: Apply width and height attributes to allow responsive image tag use loading="lazy" added width/height attributes on fallback images. If the responsive_image variables are missing either of these keys they are stripped at the very start of the preprocessor

  if (isset($variables['width']) && empty($variables['width'])) {
    unset($variables['width']);
    unset($variables['height']);
  }
  elseif (isset($variables['height']) && empty($variables['height'])) {
    unset($variables['width']);
    unset($variables['height']);
  }

Steps to reproduce

Render something with

$build = [
  '#theme' => 'responsive_image_formatter',
  '#responsive_image_style_id' => 'some_id',
  '#uri' => 'blah',
];

And do not add width or height variables.

Proposed resolution

NULL checks on the fallback.