Fix bug in lua_ui/content.lua: getmetatable(ui.content{}) shouldn't return a global mutable table

apps/openmw_test_suite/lua/test_ui_content.cpp

@@ -42,7 +42,7 @@ namespace
         mLuaState.sol()["M"] = makeContent(makeTable()).getMetatable();
         std::string testScript = R"(
             assert(not pcall(function() setmetatable(makeContent{}, {}) end), 'Metatable is not protected')
-            assert(getmetatable(makeContent{}) ~= M, 'Metatable is not protected')
+            assert(getmetatable(makeContent{}) == false, 'Metatable is not protected')
         )";
         EXPECT_NO_THROW(mLuaState.sol().safe_script(testScript));
     }

components/lua_ui/content.lua

@@ -135,6 +135,6 @@ M.__pairs = function(self)
     return next, self, 1
 end
 M.__ipairs = M.__pairs
-M.__metatable = {}
+M.__metatable = false
 
 return M