Image sanitizer only verifies if size (width/height) is not set, but doesn't validate empty values

[rcstr]

This piece validates is param is set, but in some cases images size is being set with an empty value which this function wouldn't validate. Empty values caused AMP validator to fail

// Try to extract dimensions for the image, if not set.
if ( ! isset( $new_attributes['width'] ) || ! isset( $new_attributes['height'] ) ) {
  $dimensions = AMP_Image_Dimension_Extractor::extract( $new_attributes['src'] );
    if ( is_array( $dimensions ) ) {
      $new_attributes['width'] = $dimensions[0];
      $new_attributes['height'] = $dimensions[1];
    }
}

[westonruter]

This should be fixed by #979. Please double-check.