apache-httpd: fix nix evaluation error

This only sets the timezone when it's not null to prevent:

  error: cannot coerce null to a string, at
  nixpkgs/nixos/modules/services/web-servers/apache-httpd/default.nix:676:7

nixos/modules/services/web-servers/apache-httpd/default.nix

@@ -676,6 +676,7 @@ in
       ''
         ; Needed for PHP's mail() function.
         sendmail_path = sendmail -t -i
+      '' + optionalString (!isNull config.time.timeZone) ''
 
         ; Apparently PHP doesn't use $TZ.
         date.timezone = "${config.time.timeZone}"