✨Allow special keyword "null" in assign user

[fank]

This pull request introduces a change to the assignIssue function in jira/internal/issue_impl.go to handle a special case for unassigning issues. The key update ensures that passing the string "null" as the accountID results in sending a JSON null value in the request payload, improving the clarity and functionality of the API.

Key change:

• jira/internal/issue_impl.go: Updated the assignIssue function to handle the special case where "null" is passed as the accountID. This now sends a JSON null value in the payload to unassign the issue, with a note to allow passing a nil value in the next major version.

Fixes #355

[Copilot]

Pull Request Overview

This PR updates the assignIssue function to enable handling the special case where the accountID "null" triggers unassignment by sending a JSON null payload.

• Introduces conditional payload formation based on the accountID value.
• Includes a TODO note for future support of a direct nil value.

[fank]

Other possibility

Suggested change

	if accountID == "null" {
	// Special case: if "null" is passed, send a JSON null value to unassign the issue
	// TODO: allow the user to pass a nil value in next major version
	payload = map[string]interface{}{"accountId": nil}
	if accountID == "" {
	// Special case: if an empty string is passed, send a JSON null value to unassign the issue
	// TODO: allow the user to pass a nil value in next major version
	payload = map[string]interface{}{"accountId": nil}

[fank]

@oogali what do you think?

[oogali]

@fank I think if there's a special value it can be null as you've suggested, or something more explicit and context-specific along the lines of unassigned.

I'm also okay with an empty string, however I think that would need a bit stronger, clearer language in the documentation to reflect that behavior.