Call EnsureNoError after evaluating aws_route expression so unevaluable expressions are skipped

[jandersen-plaid]

runner.EvaluateExpr can return errors that are skippable (for example, ErrUnevaluable which occurs when an expression cannot be evaluated) and entirely normal.

For example, the snippet

data "aws_transit_gateway" "tgw" {
  tags = {
    Name = "Transit Gateway"
  }
}

resource "aws_route" "tgw_route" {
  route_table_id     = "rtb-1234abcd"
  transit_gateway_id = data.aws_transit_gateway_id.tgw.id
}

Will produce the error:

Failed to check ruleset; Failed to check `aws_route_not_specified_target` rule: unevaluable expression found in example.tf:9

when the terraform is perfectly valid (and obeys the rule).

This PR just wraps the IsNull increment in a runner.EnsureNoError call so that skippable errors are skippable.

[jandersen-plaid]

Let me know if there is a preferred procedure for submitting these pulls as well (that is: creating an issue first is required, and then a PR, or something else)! I just noticed this while working through some upgrades and thought I would fix it.

[wata727]

Thanks!