Add necessary condition to enable build button

web/app/static/js/nopaque/displays/CorpusDisplay.js

@@ -55,7 +55,7 @@ class CorpusDisplay extends RessourceDisplay {
       }
     }
     for (let element of this.displayElement.querySelectorAll('.build-corpus-trigger')) {
-      if (status === 'unprepared') {
+      if (status === 'unprepared' && Object.values(this.corpus.files).length > 0) {
         element.classList.remove('disabled');
       } else {
         element.classList.add('disabled');