HTTPD: Check for SSL Cert value before building with SSL support.

HTTPD: Ensure user specifies SSL Cert and SSL Key paths when enabling SSL.

I want to fix this error. My NixOS instance had no SSL certs when I enabled SSL and rebuilt.
The build succeeded, but the HTTPD didn't start. The error? Pasted below.

"[error] Server should be SSL-aware but has no certificate configured [Hint: SSLCertificateFile]"

An assertion won't completely prevent this error, but it is helpful.