fix: use try on length(var.subnets)

[nitrocode]

what

• fix: use try on length(var.subnets)

why

• If the length is used without a try, if subnets are not passed in, the module will fail
• The use case is when the rds component is deployed with enabled = false, it relies on the vpc component which may not be deployed yet. So the enabled: false flag is used for both the vpc and rds, however rds will still try to retrieve from the remote state vpc which will try to pass in a null list of subnets which causes this failure
• A local plan against this branch results in the correct plan

references

N/A

[nitrocode]

/terratest

[nitrocode]

/terratest