Do not fail if an Exception has no backtrace

Currently `backtrace` of an exception is checked by the `empty?` method. But it sometimes occur that the Exception class will return `nil` for backtrace:
```
StandardError.new.backtrace.empty?
NoMethodError: undefined method `empty?' for nil:NilClass
```

Author: zinovyev

lib/rspec/core/formatters/exception_presenter.rb

@@ -55,7 +55,7 @@ def formatted_cause(exception)
                 cause << "  #{line}"
               end
 
-              unless last_cause.backtrace.empty?
+              unless last_cause.backtrace && last_cause.backtrace.empty?
                 cause << ("  #{backtrace_formatter.format_backtrace(last_cause.backtrace, example.metadata).first}")
               end
             end