fix: added default value to the AppConfig to prevent crashes in some cases

[k1rill]

Fix based on the crash report:

PreferencesManager.getAppConfig
Caused by java.lang.NullPointerException: fromJson(...) must not be null
       at org.openedx.app.data.storage.PreferencesManager.getAppConfig(PreferencesManager.kt:20)
       at org.openedx.profile.presentation.settings.SettingsViewModel.getConfiguration(SourceFile:7)
       at org.openedx.profile.presentation.settings.SettingsViewModel.<init>(SourceFile:32)
       at org.openedx.app.di.ScreenModuleKt$screenModule$1$28.invoke(ScreenModule.kt:193)
       at org.openedx.app.di.ScreenModuleKt$screenModule$1$28.invoke(ScreenModule.kt:193)
       at org.koin.core.instance.InstanceFactory.create(InstanceFactory.kt:53)

Steps to reproduce:

• Fresh install app
• Login into app
• See What's new
• Turn off internet connection
• Go to profile and open settings
• App crashing