JSpecify incompatibility #904

Describe the bug:
With the 45.0.0 release, some transient libraries were migrated to use JSpecify annotations by simply just adding @NullMarked to the classes.

Steps to Reproduce:

• Code that implements com.google.auth.oauth2.TokenStore.
• Code that passes a null baseUri to com.google.auth.oauth2.UserAuthorizer#getAuthorizationUrl or com.google.auth.oauth2.UserAuthorizer#getAndStoreCredentialsFromCode.

Expected behavior:

• The return value of com.google.auth.oauth2.TokenStore#load value should be @nullable, because null is the expected result for "no token found".
• The baseUri parameter in com.google.auth.oauth2.UserAuthorizer#getAuthorizationUrl and com.google.auth.oauth2.UserAuthorizer#getAndStoreCredentialsFromCode should be @nullable, because it is just required if using relative callback URIs.
• JSpecify migration made reasonable by checking what needs @nullable and what does not.

Client library version and API version:
Client library version: 45.0.0
Google Ads API version: 25
JDK version: 26