fix #782 elasticache crash on address or nil pointer dereference in case we have a single node cluster.

[wakeful]

Description

running nuke on single node elasticache cluster fails with address or nil pointer dereference

Fixes #782

TODOs

Read the Gruntwork contribution guidelines.

• Update the docs.
• Run the relevant tests successfully, including pre-commit checks.
• Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• Include release notes. If this PR is backward incompatible, include a migration guide.
• Attention Grunts - if this PR adds support for a new resource, ensure the nuke_sandbox and nuke_phxdevops jobs in .circleci/config.yml have been updated with appropriate exclusions (either directly in the job or via the .circleci/nuke_config.yml file) to prevent nuking IAM roles, groups, resources, etc that are important for the test accounts.

Release Notes (draft)

[x] Fixes #782 single node elasticache cluster fails with address or nil pointer dereference

Migration Guide

n/a

[james03160927]

LGTM