@Nullable annotation is missing at ListOperations::indexOf

[amaembo]

Methods indexOf and lastIndexOf here are documented as nullable but not annotated correspondingly.

The documentation says:

@return {@literal null} when used in pipeline / transaction or when not contained in list.

However, the methods are not annotated as @Nullable, and the package is annotated as @NonNullApi. This causes false-positive static analysis warnings. Sorry if I'm missing something.

[jxblum]

For SD Redis ListOperations, corresponding to Redis List commands, the indexOf(key, value) and lastIndexOf(key, value) methods both map to the LPOS Redis command, and I can verify the documentation states that nil will be returned when no match is found.

Therefore, it is correct to annotate our API with the Spring @Nullable annotation and make the declaration consistent with our Javadoc.