[mysql_auth & liquid_security] Update and fix

- Fix liquid_security to send an empty string to `minetest.deserialize`
   in case our file object reads `nil`.
 - Update mysql_auth from upstream to work again.

mods/liquid_security/init.lua

@@ -6,7 +6,7 @@ local timers = {}
 
 file = io.open(minetest.get_worldpath() .. "/liquid_security.dat", "r")
 if file then
-	timers = minetest.deserialize(file:read()) or {}
+	timers = minetest.deserialize(file:read() or "") or {}
 end
 
 minetest.register_on_newplayer(function(player)

mods/mysql_auth

@@ -1 +1 @@
-Subproject commit 79699b500cf0080ce281d218361dd49a1c3a8aa2
+Subproject commit 67dc261bd0e9679b01bae1854218cb442724b979