Add a default value for existing Image alt field
Add a node_preprocess function to your theme file. The logic would be to check if the current node has a value in the field for "image alt text", if not then set a default value, else return. 

These are not the exact field names but this should get you started on what to look for in your code.  

 function MYTHEME_preprocess_article(&amp;$variables) {
      if (!field_image_alt_text-&gt;getValue().strlen() = 0 || null ) { set value to something } else {return;}
};