storage: signed url with STORAGE_EMULATOR_HOST get no credentials error

**Client**

Storage

**Environment**

local emulator

**Go Environment**

$ go 1.21

**Expected behavior**

Storage client with emulator host should bypass credentials check

**Actual behavior**

When storage client is created creds will be nil (storage.co NewClient)
But when asking signedUrl in bucket.go it tries to evaluate GoogleAccessId.

For example in same situation for pubsub emulator I can use fake google credentials json. But in storage implementation it skipped.

Although I can Add access id as in options. I want to create integration test that uses exact config as in prod. so I do not want ot add opts that are not used in prod.