[TME] Nested configs are not read properly

[kimden]

A server config that includes a base config with game mode 3 and doesn't specify game mode by itself, produces game mode 3 in my upstream repo.

While setting up a server for TME using the same configuration of nested configs, it turned out that the code produces game mode 0 that corresponds to racing GP, most likely for reasons of 0 being the default value for integer parameters (but not for this one - the specific default value for game mode is 3).

Server itself works fine with the game mode value being overridden by nested config, however, that is not the intended behaviour, and it's likely that other important options are also taken from data type default values, possibly leading to unexpected behaviour.

Server version: 2e9c160

[Nomagno]

This one's on me, I touched the code without considering how it'd interact with the rest of the codebase. However, I only partially understand how to fix it 😭