Handle corrupted preferences file

Partial fix dcs-liberation/dcs_liberation#2747
Includes pydcs bump since it contains the other half of the fix.

qt_ui/liberation_install.py

@@ -36,7 +36,7 @@ def init():
                 "ignore_empty_install_directory", False
             )
             is_first_start = False
-        except KeyError:
+        except (KeyError, json.JSONDecodeError):
             __dcs_saved_game_directory = ""
             __dcs_installation_directory = ""
             __last_save_file = ""