Add @ConfigDefault("null") to Optional<String> S3 fields

- Add @ConfigDefault("null") annotations to s3_bucket, s3_prefix, s3_region, s3_access_key_id, s3_secret_access_key
- This is required for Embulk's config system to properly handle Optional fields with null defaults
- Fixes 'Field is required but not set' error when S3 configuration is not provided

Author: kentoyoshida

src/main/java/org/embulk/output/SnowflakeOutputPlugin.java

@@ -98,18 +98,23 @@ public interface SnowflakePluginTask extends PluginTask {
     public boolean getUploadJdbcLogToS3();
 
     @Config("s3_bucket")
+    @ConfigDefault("null")
     public Optional<String> getS3Bucket();
 
     @Config("s3_prefix")
+    @ConfigDefault("null")
     public Optional<String> getS3Prefix();
 
     @Config("s3_region")
+    @ConfigDefault("null")
     public Optional<String> getS3Region();
 
     @Config("s3_access_key_id")
+    @ConfigDefault("null")
     public Optional<String> getS3AccessKeyId();
 
     @Config("s3_secret_access_key")
+    @ConfigDefault("null")
     public Optional<String> getS3SecretAccessKey();
 
     public void setCopyIntoTableColumnNames(String[] columnNames);