Fix: Add null check for context parameter in XenditException constructor

[prjanitor]

Summary

This PR fixes a null pointer vulnerability in the XenditException class where the context parameter could be null, causing NullPointerException when client code attempts to iterate or access the context map.

Changes

1. Added import for java.util.Collections
2. Modified the two-argument constructor to check if context is null and initialize it to Collections.emptyMap() if so

Before

public XenditException(String message, String code, Map<String, Object> context) {
    super(message);
    this.code = code;
    this.context = context;  // Could be null!
}

After

public XenditException(String message, String code, Map<String, Object> context) {
    super(message);
    this.code = code;
    this.context = context != null ? context : Collections.emptyMap();
}

Benefits

• getContext() will never return null
• Client code can safely iterate over the context without null checks
• Follows Java best practices using Collections.emptyMap() for immutable empty collections

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