Add empty alt tag for decorative img element

From https://en.wikipedia.org/wiki/Alt_attribute

  The W3C recommends that images that convey no information, but are
  purely decorative, be specified in CSS rather than in the HTML
  markup.[10] However, it may sometimes be necessary to include a
  decorative image as an HTML img tag. In this case, if the image truly
  does not add to the content, then a blank alt attribute should be
  included in the form of alt="".[11] This makes the page navigable for
  users of screen readers or non-graphical browsers. If (in breach of
  the standard) no alt attribute has been supplied, then browsers that
  cannot display the image will still display something there, e.g. the
  URL of the image, or a fixed text string.