Fix non-null assertion on `Converter` in `ConvertingCursor`

Assertion in `ConvertingCursor` inappropriately asserts on `Cursor` and not the `Converter` being wrapped. See [here](https://github.com/spring-projects/spring-data-redis/blob/main/src/main/java/org/springframework/data/redis/core/ConvertingCursor.java#L44).