Guard useSavedIdentity against empty identity list

The SavedIdentity refactor changed useSavedIdentity to read from an
array, but getSavedIdentity returns undefined when no identity was
saved. This crashed CountItems which calls useSavedIdentity via
'the operation finished without error' without prior identity setup.
Match old behavior by returning early when no identity exists.

Issue: ZENKO-5202

Author: delthas

tests/ctst/world/Zenko.ts

@@ -792,6 +792,9 @@ export default class Zenko extends World<ZenkoWorldParameters> {
 
     useSavedIdentity() {
         const last = this.getSavedIdentity();
+        if (!last) {
+            return;
+        }
         Identity.useIdentity(last.identityType, last.identityName, last.accountName);
     }