luci-proto-wireguard: fix preshared key validation

Ensure that the preshared key option remains optional.

Fixes: #3075
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

protocols/luci-proto-wireguard/htdocs/luci-static/resources/protocol/wireguard.js

@@ -3,6 +3,9 @@
 'require network';
 
 function validateBase64(section_id, value) {
+	if (value.length == 0)
+		return true;
+
 	if (value.length != 44 || !value.match(/^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$/))
 		return _('Invalid Base64 key string');