Guard against NullReference while creating HostContext

src/Runner.Common/HostContext.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Diagnostics;
@@ -220,7 +220,7 @@ public HostContext(string hostType, string logFile = null)
             var runnerFile = GetConfigFile(WellKnownConfigFile.Runner);
             if (File.Exists(runnerFile))
             {
-                var runnerSettings = IOUtil.LoadObject<RunnerSettings>(runnerFile);
+                var runnerSettings = IOUtil.LoadObject<RunnerSettings>(runnerFile, true);
                 _userAgents.Add(new ProductInfoHeaderValue("RunnerId", runnerSettings.AgentId.ToString(CultureInfo.InvariantCulture)));
                 _userAgents.Add(new ProductInfoHeaderValue("GroupId", runnerSettings.PoolId.ToString(CultureInfo.InvariantCulture)));
             }

src/Runner.Sdk/Util/IOUtil.cs

@@ -40,10 +40,19 @@ public static void SaveObject(object obj, string path)
             File.WriteAllText(path, StringUtil.ConvertToJson(obj), Encoding.UTF8);
         }
 
-        public static T LoadObject<T>(string path)
+        public static T LoadObject<T>(string path, bool required = false)
         {
             string json = File.ReadAllText(path, Encoding.UTF8);
-            return StringUtil.ConvertFromJson<T>(json);
+            if (required && string.IsNullOrEmpty(json))
+            {
+                throw new ArgumentNullException($"File {path} is empty");
+            }
+            T result = StringUtil.ConvertFromJson<T>(json);
+            if (required && result == null)
+            {
+                throw new ArgumentException("Converting json to object resulted in a null value");
+            }
+            return result;
         }
 
         public static string GetSha256Hash(string path)

src/Test/L0/Util/IOUtilL0.cs

@@ -1,4 +1,3 @@
-using GitHub.Runner.Common.Util;
 using GitHub.Runner.Sdk;
 using System;
 using System.IO;
@@ -931,6 +930,36 @@ public void ValidateExecutePermission_ExceedsFailsafe()
             }
         }
 
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Common")]
+        public void LoadObject_ThrowsOnRequiredLoadObject()
+        {
+            using (TestHostContext hc = new(this))
+            {
+                Tracing trace = hc.GetTrace();
+
+                // Arrange: Create a directory with a file.
+                string directory = Path.Combine(hc.GetDirectory(WellKnownDirectory.Bin), Path.GetRandomFileName());
+
+                string file = Path.Combine(directory, "empty file");
+                Directory.CreateDirectory(directory);
+
+                File.WriteAllText(path: file, contents: "");
+                Assert.Throws<ArgumentNullException>(() => IOUtil.LoadObject<RunnerSettings>(file, true));
+
+                file = Path.Combine(directory, "invalid type file");
+                File.WriteAllText(path: file, contents: " ");
+                Assert.Throws<ArgumentException>(() => IOUtil.LoadObject<RunnerSettings>(file, true));
+
+                // Cleanup.
+                if (Directory.Exists(directory))
+                {
+                    Directory.Delete(directory, recursive: true);
+                }
+            }
+        }
+
         private static async Task CreateDirectoryReparsePoint(IHostContext context, string link, string target)
         {
 #if OS_WINDOWS