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

Changed files
+1
nixos
modules
services
web-servers
apache-httpd
+1
nixos/modules/services/web-servers/apache-httpd/default.nix
···
''
; 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}"