nrpe.write will add max_check_attempts: null which will cause nagios to break

[guoqiao]

In commit 198c0e3 ,
a new kwarg max_check_attempts=None was introduced to nrpe.Check class.
However, it doesn't check the None condition, which will always render max_check_attempts: null in check cfg and cause nagios to break with latest charmhelpers.

We need to add None check to ensure max_check_attempts only be rendered when it's not None.