fix: Stack creation panics if no project is provided in the creation options

[arybolovlev]

Description

When a new Stack is created without the mandatory Project field in the StackCreateOptions structure, the creation process panics instead of returning a validation error.

The following code will panic because the Project field is a pointer, and the validation method does not check whether it is nil:

s, err := client.Stacks.Create(context.TODO(), tfc.StackCreateOptions{
    Name: "a-stack",
})

This PR adds a nil pointer check to ensure that the ErrRequiredProject error is returned if the Project field is not set, or if it is set but Project.ID is empty.

Testing plan

I have added a unit test for the valid() method of the StackCreateOptions structure.

External links

Output from tests

Including output from tests may require access to a TFE instance. Ignore this section if you have no environment to test against.

$ go test . -timeout 1m -count 1 -v -run='Test_validStackCreateOptions'
=== RUN   Test_validStackCreateOptions
=== PAUSE Test_validStackCreateOptions
=== CONT  Test_validStackCreateOptions
=== RUN   Test_validStackCreateOptions/with_empty_stack_name
=== RUN   Test_validStackCreateOptions/with_empty_project_option
=== RUN   Test_validStackCreateOptions/with_empty_project_ID
=== RUN   Test_validStackCreateOptions/with_valid_options
--- PASS: Test_validStackCreateOptions (0.00s)
    --- PASS: Test_validStackCreateOptions/with_empty_stack_name (0.00s)
    --- PASS: Test_validStackCreateOptions/with_empty_project_option (0.00s)
    --- PASS: Test_validStackCreateOptions/with_empty_project_ID (0.00s)
    --- PASS: Test_validStackCreateOptions/with_valid_options (0.00s)
PASS
ok  	github.com/hashicorp/go-tfe	0.333s

Rollback Plan

Roll back to the previous version.

Changes to Security Controls

No.

[datadog-terraform-cloud-hashicorp]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

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