Compute unconfigured hcp_group description (#844)

Why:

* When an hcp_group description is not configured in a Terraform file,
  then Terraform returns an error about the state's value of null not
  matching the provider's return value of "".

This change addresses the need by:

* Add failing test for hcpi_group description not being configured.
* Fix test by configuring the hcp_group description attribute to be
  computed.

Fixes #841

.changelog/844.txt

@@ -0,0 +1,2 @@
+```release-note:bug
+Cloud Platform: fix issue where hcp_group without a description would cause a state error

internal/provider/iam/resource_group.go

@@ -65,6 +65,7 @@ The user or service account that is running Terraform when creating an %s resour
 			},
 			"description": schema.StringAttribute{
 				Description: "The group's description - maximum length of 300 characters",
+				Computed:    true,
 				Optional:    true,
 				Validators: []validator.String{
 					stringvalidator.LengthBetween(0, 300),

internal/provider/iam/resource_group_test.go

@@ -30,6 +30,16 @@ func TestAccGroupResource(t *testing.T) {
 		PreCheck:                 func() { acctest.PreCheck(t) },
 		CheckDestroy:             testAccCheckGroupDestroy(t, groupName),
 		Steps: []resource.TestStep{
+			{
+				Config: testAccGroupConfigResourceWithoutDescription(groupName),
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckResourceAttr("hcp_group.example", "display_name", groupName),
+					resource.TestCheckResourceAttr("hcp_group.example", "description", ""),
+					resource.TestCheckResourceAttrSet("hcp_group.example", "resource_name"),
+					resource.TestCheckResourceAttrSet("hcp_group.example", "resource_id"),
+					testAccGroupExists(t, "hcp_group.example", &group),
+				),
+			},
 			{
 				Config: testAccGroupConfigResource(groupName, description),
 				Check: resource.ComposeTestCheckFunc(
@@ -75,6 +85,14 @@ func testAccGroupConfigResource(displayName, description string) string {
 `, displayName, description)
 }
 
+func testAccGroupConfigResourceWithoutDescription(displayName string) string {
+	return fmt.Sprintf(`
+	resource "hcp_group" "example" {
+		display_name = %q
+	}
+`, displayName)
+}
+
 // testAccGroupsExists queries the API and retrieves the matching
 // group.
 func testAccGroupExists(t *testing.T, resourceName string, sp *models.HashicorpCloudIamGroup) resource.TestCheckFunc {