Better support for omitting role_arn

[NathanBaulch]

what

I recently started getting warnings about a missing aws provider role_arn value.

╷
│ Warning: Missing required argument
│
│   with provider["registry.opentofu.org/hashicorp/aws"],
│   on main.tf line 46, in provider "aws":
│   46: provider "aws" {
│
│ The argument "role_arn" is required, but no definition was found.
│
│ This will be an error in a future release.
╵

This seems to be because the accepter_aws_assume_role_arn variable has a default of null but is checked against empty string, meaning the assume_role block is incorrectly included with null role_arn when the variable is omitted.

Also, I'm not using an explicit requester_aws_assume_role_arn either so it would be nice if that variable had an empty default value just like accepter_aws_assume_role_arn.

why

The warning creates unnecessary noise and will be escalated to an error in future apparently.