fix(base): Key.Expires() returns 1970-01-01 for keys with no expiration #244

Problem

b2_create_key and b2_list_keys return expirationTimestamp: null for an application key with no expiration (b2_list_keys). Blazer decodes that as 0, and millitime (base/base.go:367) converts it to time.Unix(0, 0). Both paths use it: base/base.go:2089 (create) and base/base.go:2126 (list).

So Key.Expires() returns 1970-01-01T00:00:00Z with IsZero() == false. A caller can't tell "never expires" from "expired in 1970". Code that checks k.Expires().IsZero() for "no expiration" treats every such key as expired.

Reproduction

A b2_list_keys response with "expirationTimestamp": null for a key, then Client.ListKeys: Expires() on that key returns 1970-01-01, not the zero time.Time.

Expected

A key with no expiration returns the zero time.Time from Key.Expires().

Acceptance criteria

• A null or missing expirationTimestamp maps to time.Time{} in both the create and the list paths.
• A wire test with the documented null fixture asserts Expires().IsZero(), and it fails before the fix.
• The Key.Expires() godoc says what the zero value means.
• Changelog entry under Fixed.

Out of scope

Other millitime call sites (upload timestamps and so on), unless the same null case applies there; note them if found.

Links

• Found by test: add credential-free tests for calls only covered by live tests #231 (test: credential-free tests for calls only covered by live tests #203), which uses the documented null fixture and deliberately doesn't assert the wrong 1970 value.