[msbuild] Fix null check when validating Hot Restart app identifier (#18546)

The detection of signing identities should fail if we could not build a
valid app id. This code originally checked for null (`AppId == null`)
and was replaced by `is not null` as part of
36af029204 (diff-ff4502eb16b65dc59bb7de2bf980024260e83d7b97fef727c9c033374f2b3533R106).

msbuild/Xamarin.iOS.Tasks.Windows/Tasks/DetectHotRestartSigningIdentity.cs

@@ -103,7 +103,7 @@ namespace Xamarin.iOS.HotRestart.Tasks {
 
 				identity.AppId = ConstructValidAppId (identity.Profile, identity.BundleId);
 
-				if (identity.AppId is not null) {
+				if (identity.AppId is null) {
 					Log.LogError ("Project bundle identifier '{0}' does not match specified provisioning profile '{1}'. Please enable Automatic Provisioning from the iOS Bundle Signing page.", identity.BundleId, ProvisioningProfile);
 					return false;
 				}