Fix [change_theme] crashing when theme= isn't specified

Leaving theme= unspecified ought to have the same effect as providing it
and setting it to an empty string. Without the check for a nil value,
however, it would result in a crash like this:

  20180519 22:31:54 error scripting/lua: lua/wml-tags.lua:922: bad argument #3 to '__newindex' (string expected, got nil)
  stack traceback:
          [C]: in metamethod '__newindex'
          lua/wml-tags.lua:922: in local 'cmd'
          lua/wml-utils.lua:145: in field 'handle_event_commands'
          lua/wml-flow.lua:6: in function <lua/wml-flow.lua:5>

changelog.md

@@ -163,6 +163,8 @@
      (issue #3050).
    * Fixed a crash when using certain invalid color= values.
    * Fixed: unit halo remained after undoing a recall (issue #3065).
+   * [change_theme] no longer causes a Lua error when theme= is not specified
+     instead of explicitly set to an empty string.
 
 ## Version 1.13.12
  ### Security fixes

data/lua/wml-tags.lua

@@ -920,7 +920,13 @@ function wml_actions.reset_fog(cfg)
 end
 
 function wesnoth.wml_actions.change_theme(cfg)
-	wesnoth.game_config.theme = cfg.theme
+	local new_theme = cfg.theme
+
+	if new_theme == nil then
+		new_theme = ""
+	end
+
+	wesnoth.game_config.theme = new_theme
 end
 
 function wesnoth.wml_actions.zoom(cfg)