[Group 4] Enable nullable annotations for `Microsoft.Extensions.Logging.Debug` (#66862)

Related to #43605

src/libraries/Microsoft.Extensions.Logging.Debug/ref/Microsoft.Extensions.Logging.Debug.csproj

 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFrameworks>$(NetCoreAppCurrent);$(NetCoreAppMinimum);netstandard2.0;$(NetFrameworkMinimum)</TargetFrameworks>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>

src/libraries/Microsoft.Extensions.Logging.Debug/src/DebugLogger.cs

@@ -23,7 +23,7 @@ public DebugLogger(string name)
         }
 
         /// <inheritdoc />
-        public IDisposable BeginScope<TState>(TState state)
+        public IDisposable BeginScope<TState>(TState state) where TState : notnull
         {
             return NullScope.Instance;
         }
@@ -37,7 +37,7 @@ public bool IsEnabled(LogLevel logLevel)
         }
 
         /// <inheritdoc />
-        public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception exception, Func<TState, Exception, string> formatter)
+        public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception? exception, Func<TState, Exception?, string> formatter)
         {
             if (!IsEnabled(logLevel))
             {

src/libraries/Microsoft.Extensions.Logging.Debug/src/Microsoft.Extensions.Logging.Debug.csproj

@@ -2,6 +2,7 @@
 
   <PropertyGroup>
     <TargetFrameworks>$(NetCoreAppCurrent);$(NetCoreAppMinimum);netstandard2.0;$(NetFrameworkMinimum)</TargetFrameworks>
+    <Nullable>enable</Nullable>
     <EnableDefaultItems>true</EnableDefaultItems>
     <!-- Use targeting pack references instead of granular ones in the project file. -->
     <DisableImplicitAssemblyReferences>false</DisableImplicitAssemblyReferences>