Inconsistent Null Handling in JsonExtractScalar and JsonExtractIndex.

### Summary

With `SET enableNullHandling = true`, queries that use `jsonExtractScalar(...)` (no default value argument) over JSON columns where some rows have no value at the queried path fail with `IllegalArgumentException("Cannot resolve JSON path on some records. Consider setting a default value.")`. The same shape with `jsonExtractIndex(...)` likewise throws (`RuntimeException("Cannot resolve JSON path...")`).

Both `JsonExtractScalarTransformFunction` and `JsonExtractIndexTransformFunction` have `_nullHandlingEnabled` plumbed through `init()` but the six typed SV methods (`transformTo{Int,Long,Float,Double,BigDecimal,String}ValuesSV`) ignore it and throw unconditionally.

Meanwhile, `JsonIndexDistinctOperator` (which `SELECT DISTINCT jsonExtractIndex(...)` routes through when a JSON index is present) does honor null handling — it folds unresolved rows into a single SQL `NULL` group via `handleMissingDocs`. This creates a real divergence: same logical query, different outcomes based on whether the DISTINCT operator route is eligible.

### Details

A `clicks` table with a single JSON column:

```
row 0: {"country": "US", "clicks": 5}
row 1: {"country": "CA", "clicks": 3}
row 2: {"country": null, "clicks": null}
row 3: {}
```

JSON index is present on `json`.

**Today on master — `enableNullHandling = true`**

| # | Query shape | Default value | `jsonExtractScalar(json, '\$.country', 'STRING'[, …])` | `jsonExtractIndex(json, '\$.country', 'STRING'[, …])` | Consistent? |
|---|---|---|---|---|---|
| 1 | **Projection**
`SELECT FROM clicks` | none | throws `IllegalArgumentException("Cannot resolve JSON path on some records…")`
*scalar transform fn* | throws `RuntimeException("Cannot resolve JSON path…")`
*index transform fn* | ✓ both throw |
| 2 | **DISTINCT**
`SELECT DISTINCT FROM clicks` | none | throws (same scalar transform path as row 1) | returns `"US"`, `"CA"`, `NULL`
*via `JsonIndexDistinctOperator.handleMissingDocs` → `addNull()`* | ✗ **inconsistent** |
| 3 | **GROUP BY + COUNT**
`SELECT AS c, COUNT(*) FROM clicks GROUP BY c` | none | throws (same scalar transform path as row 1) | throws (same index transform path as row 1) | ✓ both throw |
| 4 | **Projection** | `'foobar'` | returns `"US"`, `"CA"`, `"foobar"`, `"foobar"` | returns `"US"`, `"CA"`, `"foobar"`, `"foobar"` | ✓ |
| 5 | **DISTINCT** | `'foobar'` | returns `"US"`, `"CA"`, `"foobar"` | returns `"US"`, `"CA"`, `"foobar"`
*via `JsonIndexDistinctOperator.handleMissingDocs` → `addValueToDistinctTable("foobar", …)`* | ✓ |
| 6 | **GROUP BY** | `'foobar'` | returns `("CA",1)`, `("US",1)`, `("foobar",2)` | returns `("CA",1)`, `("US",1)`, `("foobar",2)` | ✓ |

**After fix — `enableNullHandling = true`**

Both `JsonExtractScalarTransformFunction` and `JsonExtractIndexTransformFunction` patched to honor `_nullHandlingEnabled` when no default literal is supplied.

| # | Query shape | Default value | `jsonExtractScalar(json, '\$.country', 'STRING'[, …])` | `jsonExtractIndex(json, '\$.country', 'STRING'[, …])` | Consistent? |
|---|---|---|---|---|---|
| 1 | **Projection** | none | returns `"US"`, `"CA"`, `NULL`, `NULL`
*scalar transform fn — new null-emit branch* | returns `"US"`, `"CA"`, `NULL`, `NULL`
*index transform fn — new null-emit branch* | ✓ |
| 2 | **DISTINCT** | none | returns `"US"`, `"CA"`, `NULL`
*scalar SV result (with null bitmap) deduped by generic distinct operator* | returns `"US"`, `"CA"`, `NULL`
*via `JsonIndexDistinctOperator.handleMissingDocs` → `addNull()` (unchanged)* | ✓ |
| 3 | **GROUP BY + COUNT** | none | returns `("CA",1)`, `("US",1)`, `(NULL,2)`
*scalar SV result (with null bitmap), then standard GROUP BY* | returns `("CA",1)`, `("US",1)`, `(NULL,2)`
*index transform fn SV result (with null bitmap), then standard GROUP BY* | ✓ |
| 4 | **Projection** | `'foobar'` | returns `"US"`, `"CA"`, `"foobar"`, `"foobar"`
*unchanged — default-value branch* | returns `"US"`, `"CA"`, `"foobar"`, `"foobar"`
*unchanged — default-value branch* | ✓ |
| 5 | **DISTINCT** | `'foobar'` | returns `"US"`, `"CA"`, `"foobar"`
*unchanged* | returns `"US"`, `"CA"`, `"foobar"`
*unchanged* | ✓ |
| 6 | **GROUP BY** | `'foobar'` | returns `("CA",1)`, `("US",1)`, `("foobar",2)`
*unchanged* | returns `("CA",1)`, `("US",1)`, `("foobar",2)`
*unchanged* | ✓ |

**Change summary (NH = ON):** rows 1–3 in both function-name columns flip from `throws` to a SQL `NULL` (or null group / null row). Rows 4–6 are unchanged. Every cell is consistent across both function names post-fix.

**Today on master — `enableNullHandling = false`**

| # | Query shape | Default value | `jsonExtractScalar(...)` | `jsonExtractIndex(...)` | Consistent? |
|---|---|---|---|---|---|
| 1 | **Projection** | none | throws `IllegalArgumentException` | throws `RuntimeException` (index transform fn) | ✓ both throw |
| 2 | **DISTINCT** | none | throws (scalar transform) | throws `RuntimeException("Illegal Json Path: [...], for some docIds in segment [...]")`
*via `JsonIndexDistinctOperator.handleMissingDocs` — NH-off, no-default branch* | ✓ both throw |
| 3 | **GROUP BY** | none | throws (scalar transform) | throws (index transform) | ✓ both throw |
| 4 | **Projection** | `'foobar'` | returns `"US"`, `"CA"`, `"foobar"`, `"foobar"` | returns `"US"`, `"CA"`, `"foobar"`, `"foobar"` | ✓ |
| 5 | **DISTINCT** | `'foobar'` | returns `"US"`, `"CA"`, `"foobar"` | returns `"US"`, `"CA"`, `"foobar"` | ✓ |
| 6 | **GROUP BY** | `'foobar'` | returns `("CA",1)`, `("US",1)`, `("foobar",2)` | returns `("CA",1)`, `("US",1)`, `("foobar",2)` | ✓ |

With NH off, all cells are already consistent on master — the fix preserves the legacy throw for the no-default case (gated on `_nullHandlingEnabled`).

### Proposed fix

In both `JsonExtractScalarTransformFunction` and `JsonExtractIndexTransformFunction`:

1. When `_nullHandlingEnabled = true` and no default value was supplied: emit SQL `NULL` (write the type's null placeholder + mark the row in `getNullBitmap()`) for unresolved rows, instead of throwing.
2. When `_nullHandlingEnabled = false`: preserve the legacy throw.
3. MV transforms unchanged.
4. `JsonIndexDistinctOperator` already handles this correctly; no change needed.

**Out of scope (separate follow-up)**

- **4-arg with SQL `NULL` literal as default:** `LiteralContext.getStringValue()` returns `""` for SQL `NULL`, causing `JsonIndexDistinctOperator` to insert an empty string instead of a null group. Separate, pre-existing inconsistency.