Make EcrScanEvent severities optional

[RafalSumislawski]

Issue #, if available:

Description of changes:

Makes severities in EcrScanEventFindingSeverityCounts optional.

Quoting the documentation:

The finding-severity-counts parameter will only return a value for a severity level if one exists

I used the example from the documentation as a test for this change.

By submitting this pull request

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I confirm that I've made a best effort attempt to update all relevant documentation. (couldn't find any relevant doc in the repo)

[calavera]

Thanks!