Fix previous commit

The value should be null to always get all cookies and not only first-party isolation cookies.

From https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/cookies:
"When first-party isolation is off, the firstPartyDomain parameter is optional and defaults to an empty string.  A non-empty string can be used to retrieve or modify first-party isolation cookies."