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

Author: lqiu96

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

@@ -49,6 +49,7 @@
 import java.net.URL;
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.Date;
 import java.util.List;
 import java.util.Map;
@@ -289,7 +290,7 @@ public UserCredentials getCredentials(String userId) throws IOException {
    * @throws IOException An error from the server API call to get the tokens.
    */
   public UserCredentials getCredentialsFromCode(String code, @Nullable URI baseUri) throws IOException {
-    return getCredentialsFromCode(code, baseUri, null);
+    return getCredentialsFromCode(code, baseUri, Collections.emptyMap());
   }
 
   /**
@@ -305,6 +306,9 @@ public UserCredentials getCredentialsFromCode(String code, @Nullable URI baseUri
   public UserCredentials getCredentialsFromCode(
       String code, @Nullable URI baseUri, @Nullable Map<String, String> additionalParameters)
       throws IOException {
+    if (additionalParameters == null) {
+      additionalParameters = Collections.emptyMap();
+    }
     TokenResponseWithConfig tokenResponseWithConfig =
         getCredentialsFromCodeInternal(code, baseUri, additionalParameters);
     return UserCredentials.newBuilder()
@@ -331,6 +335,9 @@ public UserCredentials getCredentialsFromCode(
    */
   public TokenResponseWithConfig getTokenResponseFromAuthCodeExchange(
       String code, @Nullable URI callbackUri, @Nullable Map<String, String> additionalParameters) throws IOException {
+    if (additionalParameters == null) {
+      additionalParameters = Collections.emptyMap();
+    }
     return getCredentialsFromCodeInternal(code, callbackUri, additionalParameters);
   }
 
@@ -451,7 +458,7 @@ protected void monitorCredentials(String userId, UserCredentials credentials) {
   }
 
   private TokenResponseWithConfig getCredentialsFromCodeInternal(
-      String code, @Nullable URI baseUri, @Nullable Map<String, String> additionalParameters) throws IOException {
+      String code, @Nullable URI baseUri, Map<String, String> additionalParameters) throws IOException {
     Preconditions.checkNotNull(code);
     URI resolvedCallbackUri = getCallbackUri(baseUri);
 
@@ -461,10 +468,8 @@ private TokenResponseWithConfig getCredentialsFromCodeInternal(
     tokenData.put("redirect_uri", resolvedCallbackUri);
     tokenData.put("grant_type", "authorization_code");
 
-    if (additionalParameters != null) {
-      for (Map.Entry<String, String> entry : additionalParameters.entrySet()) {
-        tokenData.put(entry.getKey(), entry.getValue());
-      }
+    for (Map.Entry<String, String> entry : additionalParameters.entrySet()) {
+      tokenData.put(entry.getKey(), entry.getValue());
     }
 
     if (pkce != null) {