fix: Modify coalesce ami and ami_ssm_parameter function from always failing when ami var defined

[Zonny]

… always failing when ami var defined

Description

Include separate try functions for both var.ami and nonsensitive(data.aws_ssm_parameter.this[0].value) to allow them to return null separately instead of returning null if the whole expression fails

Motivation and Context

Change is required to fix #352

Breaking Changes

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s) (Note: I don't believe I need to make any changes to the examples for this PR)
• I have tested and validated these changes using one or more of the provided examples/* projects

• I have executed pre-commit run -a on my pull request

[antonbabenko]

This has been resolved in #354

[github-actions]

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