fix error when Steam .vdf file doesn't exist

docs/release-notes.md

@@ -7,6 +7,10 @@
   _If needed, you can update to SMAPI 3.16.0 first and then install the latest version._
 -->
 
+## Upcoming release
+* For players:
+  * Fixed installer error on Windows if the Steam library folder exists but doesn't contain Steam's `.vdf` library data file.
+
 ## 3.17.0
 Released 09 October 2022 for Stardew Valley 1.5.6 or later. See [release highlights](https://www.patreon.com/posts/73090322).
 

src/SMAPI.Toolkit/Framework/GameScanning/GameScanner.cs

@@ -264,8 +264,12 @@ private IEnumerable<string> GetCustomInstallPaths()
             if (steamPath == null)
                 return null;
 
-            // get raw .vdf data
+            // get .vdf file path
             string libraryFoldersPath = Path.Combine(steamPath.Replace('/', '\\'), "steamapps\\libraryfolders.vdf");
+            if (!File.Exists(libraryFoldersPath))
+                return null;
+
+            // read data
             using FileStream fileStream = File.OpenRead(libraryFoldersPath);
             VdfDeserializer deserializer = new();
             dynamic libraries = deserializer.Deserialize(fileStream);