Fix nullability warnings from compiler update

src/System.Private.CoreLib/shared/System/AppDomain.cs

@@ -2,7 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-#pragma warning disable CS0067 // events are declared but not used
+#pragma warning disable CS0414 // events are assigned but not used
 
 using System.Diagnostics;
 using System.IO;
@@ -67,7 +67,7 @@ public string FriendlyName
 
         public bool IsHomogenous => true;
 
-        public event EventHandler DomainUnload;
+        public event EventHandler DomainUnload = null!;
 
         public event EventHandler<FirstChanceExceptionEventArgs> FirstChanceException
         {
@@ -244,7 +244,7 @@ public event ResolveEventHandler AssemblyResolve
             remove { AssemblyLoadContext.AssemblyResolve -= value; }
         }
 
-        public event ResolveEventHandler ReflectionOnlyAssemblyResolve;
+        public event ResolveEventHandler ReflectionOnlyAssemblyResolve = null!;
 
         public event ResolveEventHandler TypeResolve
         {

src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs

@@ -4060,7 +4060,7 @@ public event EventHandler<EventSourceCreatedEventArgs> EventSourceCreated
         /// This event is raised whenever an event has been written by a EventSource for which 
         /// the EventListener has enabled events.  
         /// </summary>
-        public event EventHandler<EventWrittenEventArgs> EventWritten;
+        public event EventHandler<EventWrittenEventArgs> EventWritten = null!;
 
         static EventListener()
         {

src/System.Private.CoreLib/shared/System/Progress.cs

@@ -56,7 +56,7 @@ public Progress(Action<T> handler) : this()
         /// Handlers registered with this event will be invoked on the 
         /// <see cref="System.Threading.SynchronizationContext"/> captured when the instance was constructed.
         /// </remarks>
-        public event EventHandler<T> ProgressChanged;
+        public event EventHandler<T> ProgressChanged = null!;
 
         /// <summary>Reports a progress change.</summary>
         /// <param name="value">The value of the updated progress.</param>

src/System.Private.CoreLib/shared/System/Runtime/Loader/AssemblyLoadContext.cs

@@ -38,11 +38,11 @@ private enum InternalState
         // synchronization primitive to protect against usage of this instance while unloading
         private readonly object _unloadLock;
 
-        private event Func<Assembly, string, IntPtr> _resolvingUnmanagedDll;
+        private event Func<Assembly, string, IntPtr> _resolvingUnmanagedDll = null!;
 
-        private event Func<AssemblyLoadContext, AssemblyName, Assembly> _resolving;
+        private event Func<AssemblyLoadContext, AssemblyName, Assembly> _resolving = null!;
 
-        private event Action<AssemblyLoadContext> _unloading;
+        private event Action<AssemblyLoadContext> _unloading = null!;
 
         private readonly string? _name;