Fix to prevent recursion error in steam runner when a game is not installed

I had run across a problem with Lutris when playing around with it recently. In my case, I have Amnesia in my Lutris library but I do not have it currently installed on my system. When I attempted to launch Amnesia from Lutris, a recursion error happened as Lutris attempted to find a valid game path. Here is the snippet of where the problem occurs.

````
  File "/home/ryochan7/Sources/lutris/lutris/runners/steam.py", line 49, in get_game_path
    if self.get_game_data_path(appid):
  File "/home/ryochan7/Sources/lutris/lutris/runners/steam.py", line 62, in get_game_data_path
    steam_config = self.get_steam_config()
  File "/home/ryochan7/Sources/lutris/lutris/runners/steam.py", line 69, in get_steam_config
    return read_config(self.get_game_path())
  File "/home/ryochan7/Sources/lutris/lutris/runners/steam.py", line 49, in get_game_path
    if self.get_game_data_path(appid):
  File "/home/ryochan7/Sources/lutris/lutris/runners/steam.py", line 62, in get_game_data_path
    steam_config = self.get_steam_config()
  File "/home/ryochan7/Sources/lutris/lutris/runners/steam.py", line 69, in get_steam_config
    return read_config(self.get_game_path())
  File "/home/ryochan7/Sources/lutris/lutris/runners/steam.py", line 49, in get_game_path
    if self.get_game_data_path(appid):
````
I have made some minor modifications to the steam runner in order to get rid of this recursion problem. The changes made help fix the case where Lutris would attempt to read the global config.vdf file when there is no appmanifest file for an appid. With the changes made, Lutris will launch Steam and Steam will bring up an installer window for Amnesia in this case.