Fix exception when exception has nil backtrace

Author: janbiedermann

lib/rake/application.rb

@@ -215,7 +215,7 @@ def display_exception_details(ex) # :nodoc:
       display_exception_details_seen << ex
 
       display_exception_message_details(ex)
-      display_exception_backtrace(ex)
+      display_exception_backtrace(ex) if ex.backtrace
       display_cause_details(ex.cause) if has_cause?(ex)
     end