chore: Add NotNull extension method for IObservable

The main purpose of this is to force type `T` to not be nullable. If you
just use `Where()`, then you still get `IObservable<T?>`.

src/Common/Extensions/RxExtensions.cs

@@ -43,4 +43,11 @@ public static class RxExtensions
             }
         });
     }
+
+    // Borrowed from: https://stackoverflow.com/a/59434717/157971
+    public static IObservable<T> NotNull<T>(this IObservable<T?> observable)
+        where T : class
+    {
+        return observable.Where(x => x is not null).Select(x => x!);
+    }
 }