fix(provider): handle null parameters map in auth_login

[7d4b9]

Description

This PR fixes a crash in the auth_login block of the Vault provider when the parameters block is explicitly null.

Context

When the parameters block is set to null, Terraform crashes with:

panic: assignment to entry in nil map

The crash occurs in auth.go line 254 because the map params is not initialized before writing to it.

This PR ensures the map is initialized using make(map[string]interface{}) if parameters is nil.

Root Cause

The current implementation assumes that when .getOk("parameters") succeeds, the value is always a non-nil map. This assumption is incorrect if the user writes:

auth_login {
  method     = "gcp"
  path       = "auth/gcp/login"
  parameters = null
}

This pattern is used when switching between local and cluster-based login methods and caused us to crash in both local and remote terraform apply.

Reproducer

provider "vault" {
  address = var.vault_addr

  auth_login {
    method = "gcp"
    path   = "auth/gcp/login"
    parameters = null
  }
}

Fix

Initialize params with make(map[string]interface{}) when parameters is nil.

Checklist
	•	Added [CHANGELOG](https://github.com/hashicorp/terraform-provider-vault/blob/master/CHANGELOG.md) entry (user-facing crash fix)
	•	Acceptance tests were run against Vault OSS 1.15.0

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccProvider_authLogin*'

--- PASS: TestAccProvider_authLogin_parameters_nil (0.51s)
--- PASS: TestAccProvider_authLogin_parameters_non_nil (0.68s)
PASS
ok      github.com/hashicorp/terraform-provider-vault/internal/provider   1.234s



⸻

Community Note
	•	Please do not leave “+1” comments, they generate extra noise for pull request followers and do not help prioritize the request

[hashicorp-cla-app]

All committers have signed the CLA.

[hashicorp-cla-app]

Thank you for your submission! We require that all contributors sign our Contributor License Agreement ("CLA") before we can accept the contribution. Read and sign the agreement

Learn more about why HashiCorp requires a CLA and what the CLA includes

_{Have you signed the CLA already but the status is still pending? Recheck it.}