chore: allowed empty default for modules (#72)

From #66, also added testing for `jetbrains-gateway`.

## Regions
Azure and AWS were simple, but there is no error message when no region is selected:

https://github.com/coder/modules/assets/58410745/9a9e317b-7764-427f-b298-86313de5fb33

Open to feedback on this.

## Jetbrains Gateway
The Jetbrains gateway default IDE satisfies a ton of logic in the template, so allowing `null` would lead to a lot of patchwork `try` statements. Now it's implemented to use the first IDE in the `jetbrains_ides` as the coder_parameter default for cleanliness. 

Let me know your thoughts.

aws-region/main.test.ts

@@ -13,7 +13,7 @@ describe("aws-region", async () => {
 
   it("default output", async () => {
     const state = await runTerraformApply(import.meta.dir, {});
-    expect(state.outputs.value.value).toBe("us-east-1");
+    expect(state.outputs.value.value).toBe("");
   });
 
   it("customized default", async () => {

aws-region/main.tf

@@ -22,7 +22,7 @@ variable "description" {
 }
 
 variable "default" {
-  default     = "us-east-1"
+  default     = ""
   description = "The default region to use if no region is specified."
   type        = string
 }
@@ -131,7 +131,7 @@ data "coder_parameter" "region" {
   name         = "aws_region"
   display_name = var.display_name
   description  = var.description
-  default      = var.default
+  default      = var.default == "" ? null : var.default
   mutable      = var.mutable
   dynamic "option" {
     for_each = { for k, v in local.regions : k => v if !(contains(var.exclude, k)) }