[HotReloadAgent] fix possible NullReferenceException

[jonathanpeppers]

When running on a platform like Android, startup hooks can run via .NET's runtimeconfig.json, such as:

<ItemGroup>
  <RuntimeHostConfigurationOption Include="STARTUP_HOOKS" Value="Microsoft.Extensions.DotNetDeltaApplier" />
</ItemGroup>

If done this way, $DOTNET_STARTUP_HOOKS can be null, leading to:

01-08 16:07:49.336 14598 14598 F mono-rt :  System.NullReferenceException: Object reference not set to an instance of an object.
01-08 16:07:49.336 14598 14598 F mono-rt :    at Microsoft.DotNet.HotReload.HotReloadAgent.RemoveCurrentAssembly(Type startupHookType, String environment)
01-08 16:07:49.336 14598 14598 F mono-rt :    at Microsoft.DotNet.HotReload.HotReloadAgent.ClearHotReloadEnvironmentVariables(Type startupHookType)
01-08 16:07:49.336 14598 14598 F mono-rt :    at StartupHook.Initialize()
01-08 16:07:49.336 14598 14598 F mono-rt :    at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
01-08 16:07:49.336 14598 14598 F mono-rt :    at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)
01-08 16:07:49.336 14598 14598 F mono-rt :    --- End of inner exception stack trace ---
01-08 16:07:49.336 14598 14598 F mono-rt :    at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)
01-08 16:07:49.336 14598 14598 F mono-rt :    at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
01-08 16:07:49.336 14598 14598 F mono-rt :    at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters)
01-08 16:07:49.336 14598 14598 F mono-rt :    at System.StartupHookProvider.CallStartupHook(StartupHookNameOrPath startupHook)
01-08 16:07:49.336 14598 14598 F mono-rt :    at System.StartupHookProvider.ProcessStartupHooks(String diagnosticStartupHooks)

I think we should remove the ! and do a better null check, to just prevent this from happening.

I don't know yet if we'll use the env var or the runtimeconfig.json by default, but this seems reasonable to fix.

With the fix, it makes it to an expected state:

01-08 16:20:20.560 17468 17468 I DOTNET  : [HotReload] Environment variable DOTNET_WATCH_HOTRELOAD_NAMEDPIPE_NAME has no value

[Copilot]

Pull request overview

This PR fixes a NullReferenceException in the HotReloadAgent that occurs on platforms like Android where startup hooks can be configured via runtimeconfig.json instead of environment variables, resulting in a null DOTNET_STARTUP_HOOKS variable.

Key changes:

• Remove null-forgiving operator when retrieving environment variable
• Update RemoveCurrentAssembly to accept nullable string parameter
• Use string.IsNullOrEmpty() for more robust null/empty checking