🐛 Fix empty cookie header parsing and header value set

[AlexV525]

Fixes #1730.

New Pull Request Checklist

• I have read the Documentation
• I have searched for a similar pull request in the project and found none
• I have updated this branch with the latest main branch to avoid conflicts (via merge from master or rebase)
• I have added the required tests to prove the fix/feature I'm adding
• I have updated the documentation (if necessary)
• I have run the tests without failures
• I have updated the CHANGELOG.md in the corresponding package

[AlexV525]

An alternative change can be update the return type of CookieManager.getCookies from String to String?, but it should be considered as a breaking change, so the PR doesn't change that.

dio/plugins/cookie_manager/lib/src/cookie_mgr.dart

Lines 30 to 33 in 9951390

	/// Merge cookies into a Cookie string.
	static String getCookies(List<Cookie> cookies) {
	return cookies.map((cookie) => '${cookie.name}=${cookie.value}').join('; ');
	}

[ueman]

LGTM!

[ueman]

Good catch