While I was looking at issues
open-telemetry/opentelemetry-java-instrumentation#6694 and
open-telemetry/opentelemetry-java#2337, I saw that the code in
`io.opentelemetry.instrumentation.resources.ContainerResource` used
`null` several times as return value which isn't safe. Nowadays,
`Optional` is better suited to signal the absence of a result, so I
refactored `ContainerResource` to use `Optional`s instead of null.
On the way, I also refactored this class's unit tests into parameterised
tests to reduce test code duplication. These improvements should help
implementing a solution to
open-telemetry/opentelemetry-java-instrumentation#6694.
Co-authored-by: Trask Stalnaker <trask.stalnaker@gmail.com>
Mateusz Rzeszutek