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

Author: lqiu96

google-auth-library-java/oauth2_http/java/com/google/auth/oauth2/MemoryTokensStorage.java

@@ -35,7 +35,6 @@
 import java.util.HashMap;
 import java.util.Map;
 import org.jspecify.annotations.NullMarked;
-import org.jspecify.annotations.Nullable;
 
 /** Represents an in-memory storage of tokens. */
 @NullMarked

google-auth-library-java/oauth2_http/java/com/google/auth/oauth2/TokenStore.java

@@ -46,8 +46,7 @@ public interface TokenStore {
    * @return The loaded token data.
    * @throws IOException An error loading the token data from storage.
    */
-  @Nullable
-  String load(String id) throws IOException;
+  @Nullable String load(String id) throws IOException;
 
   /**
    * Put the token data into storage for the given ID.