fix(auth): fix JSpecify nullability in UserAuthorizer and TokenStore

[lqiu96]

This PR is part 1 of 3 in a stacked series of JSpecify nullability fixes for google-auth-library-java:

1. This PR (fix(auth): fix JSpecify nullability in UserAuthorizer and TokenStore #14150): fix(auth): fix JSpecify nullability in UserAuthorizer and TokenStore (resolves JSpecify incompatibility #14147)
2. fix(auth): add missing @Nullable annotations across credential types and providers #14153: fix(auth): add missing @Nullable annotations across credential types and providers
3. fix(auth): annotate builder fields, setters, and getters as @Nullable across credential builders #14154: fix(auth): annotate builder fields, setters, and getters as @Nullable across credential builders

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Fixes #14147

Description

In google-auth-library-java, classes were recently annotated with @NullMarked at the class level. This PR specifically addresses the incompatibilities reported in #14147:

1. TokenStore#load and MemoryTokensStorage#load: Annotates the return type of load(String id) with @Nullable, as null is the expected result when no token data is found for a given identifier.
2. UserAuthorizer#getAuthorizationUrl and UserAuthorizer#getAndStoreCredentialsFromCode: Annotates the baseUri parameter with @Nullable across getAuthorizationUrl, getAndStoreCredentialsFromCode, getCredentialsFromCode, getTokenResponseFromAuthCodeExchange, and getCallbackUri, as baseUri is optional and only required when using relative callback URIs.

[gemini-code-assist]

Code Review

This pull request comprehensively adds @nullable annotations across various credential and utility classes in the google-auth-library-java library to improve null safety and static analysis. While these changes are beneficial, several issues were identified where the new nullability annotations introduce potential bugs or contradictions. Specifically, in AccessToken.java and ClientSideCredentialAccessBoundaryFactory.java, dereferencing newly nullable fields (expirationTimeMillis and universeDomain) without null checks can lead to NullPointerExceptions. Additionally, in GdchCredentials.java, annotating apiAudience as @nullable is misleading because the method explicitly rejects null values.

[lqiu96]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces @Nullable annotations across various credential and utility classes (such as AppEngineCredentials, ClientSideCredentialAccessBoundaryFactory, Credentials, AccessToken, AwsCredentials, ComputeEngineCredentials, IdTokenCredentials, UserAuthorizer, and others) to improve null-safety under @NullMarked contexts. It also adds unit tests in UserAuthorizerTest to verify behavior with null arguments. Feedback on the changes highlights two null-safety issues: a potential NullPointerException in ClientSideCredentialAccessBoundaryFactory due to calling .equals() on the newly nullable universeDomain field, and a nullability mismatch in IdTokenCredentials where the nullable targetAudience is passed to a method expecting a non-null argument.

[lqiu96]

Added these annotations because there of the null checks below (L215 and L220)

[lqiu96]

constructor creates a default one if builder passes in null

[lqiu96]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces nullability annotations (@nullable) across MemoryTokensStorage, TokenStore, and UserAuthorizer to improve null safety, simplifies map operations using Map.putAll(), and adds corresponding unit tests for null base URIs. The feedback suggests avoiding the reassignment of the additionalParameters method parameter in getCredentialsFromCode and getTokenResponseFromAuthCodeExchange by utilizing a local variable instead.