nixos/caddy: Make virtualHosts' logFormat optional

Allow `services.caddy.virtualHosts.<name>.logFormat` to be null, and in
that case, do not add a `log {}` block to the Caddy configuration. This
makes it possible to have multiple loggers declared in the global
configuration, and instruct Caddy to use a logger by name.

Signed-off-by: Gergely Nagy <[e-mail]>

nixos/modules/services/web-servers/caddy/default.nix

···
         ${optionalString (
           hostOpts.useACMEHost != null
         ) "tls ${sslCertDir}/cert.pem ${sslCertDir}/key.pem"}
-        log {
-          ${hostOpts.logFormat}
-        }
+        ${optionalString (hostOpts.logFormat != null) ''
+          log {
+            ${hostOpts.logFormat}
+          }
+        ''}
 
         ${hostOpts.extraConfig}
       }

nixos/modules/services/web-servers/caddy/vhost-options.nix

···
     };
 
     logFormat = mkOption {
-      type = types.lines;
+      type = types.nullOr types.lines;
       default = ''
         output file ${cfg.logDir}/access-${lib.replaceStrings [ "/" " " ] [ "_" "_" ] config.hostName}.log
       '';