Fix flickering when transit gateway description is not set

[mattock]

what

This PR fixes flickering on every Terraform run when var.transit_gateway_description is unset.

The problem is caused by the leading space in the description: something that AWS APIs accept without erroring out or warning, but which gets removed or ignored. Hence Terraform tries to "fix" the situation on every run.

why

Without this change this happens on every Terraform run if var.transit_gateway_description is not defined:

Terraform will perform the following actions:                                                             

  # module.transit_gateway.aws_ec2_transit_gateway.default[0] will be updated in-place                    
  ~ resource "aws_ec2_transit_gateway" "default" {   
      ~ description                     = "Transit Gateway" -> " Transit Gateway"                         
        id                              = "tgw-07987d7d0a101968b"                                         
        tags                            = {}         
        # (11 unchanged attributes hidden)           
    }                                                

Plan: 0 to add, 1 to change, 0 to destroy.           

With this change applied things work as expected:

No changes. Your infrastructure matches the configuration.

[nitrocode]

The solution is to provide at least one of the context null label inputs such as name

A better fix for people who do not want to use name would be to use a join(" ", compact([module.this.id, "Transit Gateway"])). The description should probably be its own exposed input with the same default value Transit Gateway as it is today

[Gowiem]

Suggested change

	description = var.transit_gateway_description == "" ? format("%sTransit Gateway", module.this.id) : var.transit_gateway_description
	description = var.transit_gateway_description == "" ? join(" ", compact([module.this.id, "Transit Gateway"])) : var.transit_gateway_description

[Gowiem]

@mattock Please accept the suggested change. While we understand your solution, it degrades the most common approach, which is to use the null-label inputs and combine those with the " Transit Gateway" description. @nitrocode's solution solves for both and is preferred. Once the suggested change is accepted we'll get this approved + merged.

[mergify]

Thanks @mattock for creating this pull request!

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