caddy-frontend: Defend better against malformed ssl_proxy_ca_crt

ssl_proxy_ca_crt can be just empty value, and that's not acceptable.

software/caddy-frontend/buildout.hash.cfg

@@ -26,7 +26,7 @@ md5sum = 0851faa528eb4f21330a6f23f77dea7f
 
 [template-caddy-replicate]
 filename = instance-apache-replicate.cfg.in
-md5sum = a544bf7586f5945bbf108abe9818c7dd
+md5sum = 6d7113ebf0c46b0e4c72c128ebb647db
 
 [template-slave-list]
 _update_hash_filename_ = templates/apache-custom-slave-list.cfg.in

software/caddy-frontend/instance-apache-replicate.cfg.in

@@ -141,8 +141,8 @@ context =
 {%       do slave_error_list.append('slave https-url %r invalid' % (slave['https-url'],)) %}
 {%     endif %}
 {%   endif %}
-{%   set ssl_proxy_ca_crt = slave.get('ssl_proxy_ca_crt') %}
-{%   if ssl_proxy_ca_crt %}
+{%   if 'ssl_proxy_ca_crt' in slave %}
+{%     set ssl_proxy_ca_crt = slave.get('ssl_proxy_ca_crt', '') %}
 {%     set check_popen = popen([parameter_dict['openssl'], 'x509', '-noout']) %}
 {%     do check_popen.communicate(ssl_proxy_ca_crt) %}
 {%     if check_popen.returncode != 0 %}