4.x: List injection should not fail on empty list

[tomas-langer]

Currently when we inject a List<ServiceProvider<SomeType>>, if there is no provider of SomeType available, we get an exception.
The workaround is to manually use PicoServices.realizedServices() to lookup all with expected set to false

The default behavior I think should be to use empty list if none is found, as this is going to be used for looking up extensions by users (Http services, config sources etc.) that may not be present.

This should be fixed in:
ConfigProducer (if already merged)
LoomServer - now fails if no service discovered

[trentjeff]

I'm debating whether to just fix this to "allow empty lists always", or proactively adding @InjectOptions to the API with an attribute boolean atLeastOne - thoughts on which direction to take - simple or advanced??

[tomas-langer]

Please go with the "simple" approach of allowing injection of empty lists.