MINOR: ssl: allow to start without certificate if strict-sni is set

MINOR: ssl: allow to start without certificate if strict-sni is set

With strict-sni, ssl connection will fail if no certificate match. Have no
certificate in bind line, fail on all ssl connections. It's ok with the
behavior of strict-sni. When 'generate-certificates' is set 'strict-sni' is
never used. When 'strict-sni' is set, default_ctx is never used. Allow to start
without certificate only in this case.

Use case is to start haproxy with ssl before customer start to use certificates.
Typically with 'crt' on a empty directory and 'strict-sni' parameters.