Enable nullable for HostFactoryResolver.Tests (dotnet#103609)

Author: ericstj

src/libraries/Microsoft.Extensions.HostFactoryResolver/tests/HostFactoryResolverTests.cs

@@ -271,9 +271,12 @@ public void ApplicationNameSetFromArgument()
         {
             Assembly assembly = Assembly.Load("ApplicationNameSetFromArgument");
             var factory = HostFactoryResolver.ResolveServiceProviderFactory(assembly, s_WaitTimeout);
+            Assert.NotNull(factory);
             IServiceProvider? serviceProvider = factory(Array.Empty<string>());
+            Assert.NotNull(serviceProvider);
 
-            var configuration = (IConfiguration)serviceProvider.GetService(typeof(IConfiguration));
+            var configuration = (IConfiguration?)serviceProvider.GetService(typeof(IConfiguration));
+            Assert.NotNull(configuration);
             Assert.Equal("ApplicationNameSetFromArgument", configuration["applicationName"]);
         }
 
@@ -284,7 +287,7 @@ public void NoSpecialEntryPointPatternCanRunInParallel()
             var factory = HostFactoryResolver.ResolveServiceProviderFactory(typeof(NoSpecialEntryPointPattern.Program).Assembly, s_WaitTimeout);
             Assert.NotNull(factory);
 
-            var tasks = new Task<IServiceProvider>[30];
+            var tasks = new Task<IServiceProvider?>[30];
             int index = 0;
             for (int i = 0; i < tasks.Length; i++)
             {

src/libraries/Microsoft.Extensions.HostFactoryResolver/tests/Microsoft.Extensions.HostFactoryResolver.Tests.csproj

@@ -1,6 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFrameworks>$(NetCoreAppCurrent);$(NetFrameworkMinimum)</TargetFrameworks>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>