fix: only send volumeConfigurations when EBS inputs are provided

[wilsonjord]

Description of changes:

Resolves a regression introduced by #721 where the action sent an empty volumeConfigurations array to UpdateService and RunTask when no inputs were provided. AWS interprets the empty array as "clear all volume configurations", rather than "no change", which was the pre-v2.3.0 behaviour.
This new behaviour causes deployment failures for services managed externally (e.g. via Terraform), as externally configured EBS volumes where cleared; EBS configuration to be redundantly declared again in the workflow to avoid this failure.

Fix: only include volumeConfigurations when inputs are provided, otherwise exclude.

Tests: removed volumeConfigurations: [] assertions that were based on the buggy behaviour. Deleted remove service EBS volume configuration test completely, as that behaviour is no longer possible.

Behaviour change: currently, users can clear a services's volume configuration by omitting the EBS inputs, which sends an explicit []. This fix removes that ability - if desired, I can follow up with the ability for a user to explicitly clear volumes (a service-managed-ebs-volume-clear-like input).

[wilsonjord]

@omkhegde thank you for your review. I notice there is a failing check CI "Send Notifications to Slack"...is there anything needed on my part?