enable_agent_tls_for_checks should throw an error if no certs defined

In our team we added `enable_agent_tls_for_checks` and set all certs in tls.https. After a ton of debugging we found out we need to set the certs in defaults also to get `enable_agent_tls_for_checks` running.

#### Feature Description

As a user of consul it would be very helpful to get an error if you use `enable_agent_tls_for_checks` and the certs are not defined properly in defaults or it should also be able to take the certs out of https.

#### Use Case(s)

Setup Consul.