fix: define project_assignment block in mongodbatlas_project_api_key as required to avoid plugin crash

[AgustinBettati]

Description

Link to any related issue(s): Not strictly related to INTMDB-1296, but helps understand how this resource is being used.

Given the current definition of the project_api_key resource, it is only usable if at least one project_assignment block is defined.

Example:

resource "mongodbatlas_project_api_key" "test" {
	project_id     = mongodbatlas_project.test.id
	description  = %[3]q
	project_assignment  {
		project_id = mongodbatlas_project.test.id
		role_names = [%[4]q]
	}
}

If project_assignment is not defined, there is no way to create an API Key as no role_names are defined. The error returned in this case is:

Error: The terraform-provider-mongodbatlas plugin crashed!
panic: runtime error: invalid memory address or nil pointer dereference

This is not considered a breaking change as it is simply improving the error message users receive if they do not define the project_assignment block.

For reference this attribute was originally defined as optional because role_names could be defined at root level. However this attribute was deleted in 1.12.0 (#1418).

Type of change:

• Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR.
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the contribution guidelines
• I have checked that this change does not generate any credentials and that they are NOT accidentally logged anywhere.
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code
• If changes include deprecations or removals, I defined an isolated PR with a relevant title as it will be used in the auto-generated changelog.

Further comments

[github-actions]

Package	Line Rate	Health
github.com/mongodb/terraform-provider-mongodbatlas/mongodbatlas	3%	❌
github.com/mongodb/terraform-provider-mongodbatlas/mongodbatlas/framework/validator	68%	➖
github.com/mongodb/terraform-provider-mongodbatlas/mongodbatlas/testutils	46%	❌
github.com/mongodb/terraform-provider-mongodbatlas/mongodbatlas/util	17%	❌
Summary	3% (328 / 10474)	❌

[lantoli]

is it useful to create/modify a test?

[AgustinBettati]

All tests/examples are already defining at least on project_assignment. Don't see much value in testing empty project_assignment as it is validated by schema + makes no real sense.