Fix inverted null check in mapToJson method

[marko-kriskovic]

Summary

This PR fixes the failing tests in JsonMapConverterTest by correcting an inverted null check in the mapToJson method.

Problem

The mapToJson method at code/src/main/java/org/example/util/JsonMapConverter.java:41 had an inverted condition:

if (map != null) {
    throw new IllegalArgumentException("Map cannot be null");
}

This caused the method to:

• Throw an exception when a valid (non-null) map was passed
• Process null maps without throwing an exception

Solution

Changed the condition to correctly check for null:

if (map == null) {
    throw new IllegalArgumentException("Map cannot be null");
}

Tests Fixed

This resolves all failing tests mentioned in issue #16:

• ✅ testMapToJson_NullInput
• ✅ testMapToJson_NestedMap
• ✅ testMapToJson_EmptyMap
• ✅ testRoundTrip_JsonToMapToJson

Fixes #16