Check ProjectAssetsFile exists before reading

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: brettfo

nuget/helpers/lib/NuGetUpdater/NuGetUpdater.Core/Discover/SdkProjectDiscovery.cs

@@ -518,6 +518,11 @@ ILogger logger
             {
                 if (propertiesForProject.TryGetValue("ProjectAssetsFile", out var assetsFilePath))
                 {
+                    if (!File.Exists(assetsFilePath))
+                    {
+                        throw new FileNotFoundException("The file specified at $(ProjectAssetsFile) does not exist.");
+                    }
+
                     var assetsContent = File.ReadAllText(assetsFilePath);
                     var assets = JsonDocument.Parse(assetsContent).RootElement;
                     return assets;