Fix NPE in RecurringTransactionController.createTransaction

As the amount parameter is optional it may be null - use the appropriate
Optional method ofNullable()

src/main/java/de/financer/controller/RecurringTransactionController.java

@@ -45,6 +45,7 @@ public class RecurringTransactionController {
 
     @RequestMapping("createTransaction")
     public ResponseEntity createTransaction(String recurringTransactionId, Long amount) {
-        return this.recurringTransactionService.createTransaction(recurringTransactionId, Optional.of(amount)).toResponseEntity();
+        return this.recurringTransactionService.createTransaction(recurringTransactionId, Optional.ofNullable(amount))
+                                               .toResponseEntity();
     }
 }