add ignoreNullValues for AWS CloudWatch Scaler

[robpickerill]

This PR adds support for erroring the AWS Cloudwatch Scaler when the GetMetricData call returns an empty set of metrics, resulting in no scale-changing behaviour when no custom metrics are available during the Cloudwatch query window.

Checklist

• [x] When introducing a new scaler, I agree with the scaling governance policy
• [x] I have verified that my change is according to the deprecations & breaking changes policy
• [x] Tests have been added
• [x] Changelog has been updated and is aligned with our changelog requirements
• [x] A PR is opened to update our Helm chart (repo) (if applicable, ie. when deployment manifests are modified)
• [x] A PR is opened to update the documentation on (repo) (if applicable)
• [x] Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes #5352

Relates to:

• https://github.com/kedacore/keda-docs/pull/1346/files

[robpickerill]

I added end to end tests here for both minMetricValue and errorWhenNullValues to cover off both scenarios. I need to tidy them up a little, which ill do over the next few days.

[zroubalik]

@robpickerill any updates here please?

[robpickerill]

@robpickerill any updates here please?

Good for review, all comments are addressed

[JorTurFer]

/run-e2e aws Update: You can check the progress here