Fix exception at startup w/o default game inst

[HebaruSan]

Problems

If you have multiple game instances and no default game instance, the Manage Game Instances window appears at startup. In v1.33.0, this exception is thrown after you select an instance:

System.ArgumentException: Empty path name is not legal.
   at System.IO.StreamWriter..ctor(String path, Boolean append, Encoding encoding, Int32 bufferSize, Boolean checkHost)
   at System.IO.StreamWriter..ctor(String path)
   at CKAN.GUI.GUIConfiguration.SaveConfiguration(GUIConfiguration configuration)
   at CKAN.GUI.Main.CurrentInstanceUpdated(Boolean allowRepoUpdate)
   at System.ComponentModel.BackgroundWorker.OnRunWorkerCompleted(RunWorkerCompletedEventArgs e)

There's also a "Check for updates" prompt that will happen every time.

(Everything works fine if "Set as default" is checked for some instance.)

Initially reported by a Discord who prefers to remain anonymous, then Poppa Wheelie on the forum.

Causes

As of #3829, when you have no default instance, we use GUIConfiguration's constructor to get the window geometry defaults from an instance of the class that doesn't correspond to a real file on disk, meaning its path field is null.

• Main.CurrentInstanceUpdated always saves the current GUIConfiguration object before loading the next one, but this causes an attempt to write to a null file path with the special instance described above
• The "Check for updates" prompt also uses the special GUIConfiguration instance, which can't save

Changes

• Now GUIConfiguration.Save will abort if GUIConfiguration.path is null or empty, which will prevent the exception from occurring
• Now the "Check for updates" prompt is moved to Main.CurrentInstanceUpdated, where it will have a valid GUIConfiguration object and therefore always be able to save your choice

Fixes #3862.

We should do a hotfix release after this merged. I'm thinking v1.33.2, "Laplace" Δ.

[HebaruSan]

@LunarHackett003 and @Kronosiesium, you can try out a dev build under the Artifacts dropdown here to confirm the fix works:

https://github.com/KSP-CKAN/CKAN/pull/3863/checks

[HebaruSan]

I would like to ship a hotfix release before this impacts more users; self-reviewing...