observatory: Ignore new static Warning

There are two spots where a nullable value is passed to a non-nullable
Future.value call, or a non-nullable Completer.complete call. In the
interest of not making any functional change, I'm just ignoring this
new code.

```
warning - lib/src/service/object.dart:870:29 - 'Future<Isolate>.value' shouldn't be called with an argument of nullable type 'Isolate?'. Try passing a non-null argument. - nullable_argument_to_non_null_type
warning - lib/src/service/object.dart:1559:29 - 'Future<Class>.value' shouldn't be called with an argument of nullable type 'Class?'. Try passing a non-null argument. - nullable_argument_to_non_null_type
```

Related to #53253

TEST=N/A

Change-Id: I091990975a1f2b927e007564b6ca0658c6a34c20
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/324762
Reviewed-by: Ben Konyi <bkonyi@google.com>
Auto-Submit: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

runtime/observatory/analysis_options.yaml

@@ -3,6 +3,7 @@ analyzer:
     dead_code: info
     getter_not_subtype_setter_types: info
     missing_enum_constant_in_switch: info
+    nullable_argument_to_non_null_type: info
     unused_element: info
     unused_field: info
     unused_import: info