BUG FIX: Classifiers might me empty (#2062)

* classifiers might me empty
* return null if stored cache is null

Author: odeimaiz

services/web/client/source/class/osparc/data/Resources.js

@@ -652,6 +652,9 @@ qx.Class.define("osparc.data.Resources", {
       } catch (err) {
         return null;
       }
+      if (stored === null) {
+        return null;
+      }
       if (typeof stored === "object" && Object.keys(stored).length === 0) {
         return null;
       }

services/web/client/source/class/osparc/store/Store.js

@@ -117,7 +117,8 @@ qx.Class.define("osparc.store.Store", {
     },
     classifiers: {
       check: "Array",
-      init: [],
+      init: null,
+      nullable: true,
       event: "changeClassifiers"
     }
   },
@@ -419,7 +420,7 @@ qx.Class.define("osparc.store.Store", {
     getAllClassifiers: function(reload = false) {
       return new Promise((resolve, reject) => {
         const oldClassifiers = this.getClassifiers();
-        if (!reload && oldClassifiers.length) {
+        if (!reload && oldClassifiers !== null) {
           resolve(oldClassifiers);
           return;
         }