Observability extension: use optionals instead of nullables

[sylvlecl]

• Do you want to request a feature or report a bug?

Feature

• What is the current behavior?

Quality objects in InjectionObservability and BranchObservability are nullable: we should probably use Optional<ObservabilityQuality> instead, to be consistent with the library guideline.

Note: if we plainly replace the current methods, that will be a breaking change.