Fix inverted null check in mapToJson method

[marko-kriskovic]

Summary

This PR fixes issue #18 where tests were failing due to an inverted null check in the mapToJson method.

Problem

The mapToJson method in JsonMapConverter.java had a logic error on line 41:

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

This condition was backwards - it was throwing an exception when the map was NOT null, instead of when it WAS null.

Impact

This bug caused 5 test failures:

1. ❌ testMapToJson_SimpleMap (line 78) - Failed because valid map triggered exception
2. ❌ testMapToJson_NestedMap (line 97) - Failed because valid map triggered exception
3. ❌ testMapToJson_EmptyMap (line 110) - Failed because valid empty map triggered exception
4. ❌ testMapToJson_NullInput (line 118) - Failed because null input did NOT trigger exception
5. ❌ testRoundTrip_JsonToMapToJson (line 144) - Failed because valid map triggered exception

Solution

Changed line 41 from:

if (map != null) {

To:

if (map == null) {

This simple fix corrects the logic so that:

• ✅ Valid maps are processed normally
• ✅ Null maps throw IllegalArgumentException as expected

Testing

All 5 previously failing tests should now pass with this fix.

Fixes #18

[marko-kriskovic]

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