gluon-mesh-vpn-fastd: fix empty string key

Co-Authored-By: Matthias Schiffer <mschiffer@universe-factory.net>

package/gluon-mesh-vpn-fastd/luasrc/usr/lib/lua/gluon/mesh-vpn/provider/fastd.lua

@@ -7,7 +7,13 @@ local vpn_core = require 'gluon.mesh-vpn'
 local M = {}
 
 function M.public_key()
-	return util.trim(util.exec('/etc/init.d/fastd show_key mesh_vpn'))
+	local key = util.trim(util.exec('/etc/init.d/fastd show_key mesh_vpn'))
+
+	if key == '' then
+		key = nil
+	end
+
+	return key
 end
 
 function M.enable(val)