fix: JSON NestedMap + add tests

[SpencerTorres]

Summary

The NestedMap function converts a flat map[string]any into a structure usable by json.Marshal. The behavior has been updated to match that of ClickHouse server. A good example of this feature in use is the Grafana integration.

The main change here is the exclusion of nil values from encoding.

Comparison

Here is the outputs before/after for comparison (the sorting is inconsistent since there's no sorting key)

ClickHouse Server (expected output):

SELECT *
FROM test2

Query id: e7dc18ee-ddd1-47ee-9263-3750f6a298c4

   ┌─json─┐
1. │ {}   │
   └──────┘
   ┌─json────────────────────────────────────────┐
2. │ {"a":{"b":"42"},"c":["1","2","3"]}          │
3. │ {"f":"Hello, World!"}                       │
4. │ {"a":{"b":"43","e":"10"},"c":["4","5","6"]} │
   └─────────────────────────────────────────────┘
   ┌─json────────────────────────────────────────┐
5. │ {"a":{"b":"42"},"c":["1","2","3"]}          │
6. │ {"f":"Hello, World!"}                       │
7. │ {"a":{"b":"43","e":"10"},"c":["4","5","6"]} │
   └─────────────────────────────────────────────┘
    ┌─json────────────────────────────────────────┐
 8. │ {"a":{"b":"42"},"c":["1","2","3"]}          │
 9. │ {"f":"Hello, World!"}                       │
10. │ {"a":{"b":"43","e":"10"},"c":["4","5","6"]} │
    └─────────────────────────────────────────────┘

10 rows in set. Elapsed: 0.001 sec. 

Grafana output (incorrect, nulls are included):

Grafana output (corrected):

Checklist

• Unit and integration tests covering the common scenarios were added