Fix null check bug in mapToJson method

[marko-kriskovic]

Summary

This PR fixes the failing tests in JsonMapConverterTest by correcting a logical error in the mapToJson method.

Problem

The mapToJson method had an inverted null check condition on line 38:

• Before: if (map != null) - throws exception when map is NOT null
• After: if (map == null) - throws exception when map IS null

This bug caused the method to throw an IllegalArgumentException for valid (non-null) map inputs, while allowing null inputs to proceed, which is the opposite of the intended behavior.

Changes

• Fixed the condition in mapToJson method from if (map != null) to if (map == null)
• This now properly validates null input and throws IllegalArgumentException as expected by the tests
• Behavior now matches the mapToPrettyJson method which already had the correct null check

Test Coverage

This fix resolves the following failing tests:

• testMapToJson_NullInput - Now correctly throws exception for null input
• testMapToJson_SimpleMap - Now works correctly with valid map input
• testMapToJson_NestedMap - Now works correctly with nested maps
• testMapToJson_EmptyMap - Now works correctly with empty maps
• testRoundTrip_JsonToMapToJson - Now completes the round-trip conversion successfully

Fixes #25