Allow default ASM credential chain

[yohamta0]

Summary

• when asm:///... is used without explicit credentials, ASM currently falls back to credentials.NewEnvCredentials()
• that provider only supports raw access keys from environment variables and bypasses the AWS SDK default credential chain
• this change leaves credentials unset in that case so the AWS SDK can resolve them through its normal default chain
• explicit URL credentials and configured static credentials are unchanged

Testing

• go test ./...