fix(misconf): perform operations on attribute safely

[nikpivkin]

In one of the PRs, we started storing unknown values in context to prevent loss of value types during evaluation. Operations on attributes with unknown values during adaptation can lead to panic. We need to perform operations on attributes safely and verify that the values are not only non-null but also known.

Discussed in #8778

[simar7]

An example to reproduce is here https://raw.githubusercontent.com/aws-samples/aws-stepfunctions-ecs-fargate-process/d748389c6ee443389a7275f9056f712f9359b178/templates/roles.tf