Deprecated warning with incomplete quiz attempts

### Reproduction Steps

1. Turn on debugging display on the site
2. Start taking a quiz in a course
3. Refresh the page to exit the quiz before completed

### Expected Behavior

No warning is displayed

### Actual Behavior

Warning is displayed in the attempts dropdown:

and when you select that attempt in the dropdown:

### Error Messages / Logs

```
PHP Deprecated: round(): Passing null to parameter #1 ($num) of type int|float is deprecated in .../wp-content/plugins/lifterlms/templates/quiz/results.php on line 75

PHP Deprecated: strtotime(): Passing null to parameter #1 ($datetime) of type string is deprecated in .../wp-content/plugins/lifterlms/includes/models/model.llms.quiz.attempt.php on line 344
```

This issue has be recreated:
+ [ ] Locally
+ [ ] On a staging site
+ [ ] On a production website
+ [ ] With only LifterLMS and a default theme