…sts testFold {TestCase=<double> #7} elastic#114175

…tic#140027)

This PR fixes the issue where `INLINE STATS GROUP BY null` was being
incorrectly pruned by `PruneLeftJoinOnNullMatchingField`.

Fixes elastic#139887

## Problem For query:

```
FROM employees
| INLINE STATS c = COUNT(*) BY n = null
| KEEP c, n
| LIMIT 3
```

During `LogicalPlanOptimizer`:

```
Limit[3[INTEGER],false,false]
\_EsqlProject[[c{r}#2, n{r}#4]]
  \_InlineJoin[LEFT,[n{r}#4],[n{r}#4]]
    |_Eval[[null[NULL] AS n#4]]
    | \_EsRelation[employees][<no-fields>{r$}#7]
    \_Aggregate[[n{r}#4],[COUNT(*[KEYWORD],true[BOOLEAN],PT0S[TIME_DURATION]) AS c#2, n{r}#4]]
      \_StubRelation[[<no-fields>{r$}#7, n{r}#4]]
```

The following join node:

```
InlineJoin[LEFT,[n{r}#4],[n{r}#4]]
|_Eval[[null[NULL] AS n#4]]
| \_EsRelation[employees][<no-fields>{r$}#7]
\_Aggregate[[n{r}#4],[COUNT(*[KEYWORD],true[BOOLEAN],PT0S[TIME_DURATION]) AS c#2, n{r}#4]]
  \_StubRelation[[<no-fields>{r$}#7, n{r}#4]]
```

should NOT have `PruneLeftJoinOnNullMatchingField` applied, because the
right side is an `Aggregate` (originating from `INLINE STATS`). Since
`STATS` supports `GROUP BY null`, the join key being null is a valid use
case. Pruning this join would incorrectly eliminate the aggregation
results, changing the query semantics.

During `LocalLogicalPlanOptimizer`:

```
ProjectExec[[c{r}#2, n{r}#4]]
\_LimitExec[3[INTEGER],null]
  \_ExchangeExec[[c{r}#2, n{r}#4],false]
    \_FragmentExec[filter=null, estimatedRowSize=0, reducer=[], fragment=[<>
Project[[c{r}#2, n{r}#4]]
\_Limit[3[INTEGER],false,false]
  \_InlineJoin[LEFT,[n{r}#4],[n{r}#4]]
    |_Eval[[null[NULL] AS n#4]]
    | \_EsRelation[employees][<no-fields>{r$}#7]
    \_LocalRelation[[c{r}#2, n{r}#4],Page{blocks=[LongVectorBlock[vector=ConstantLongVector[positions=1, value=100]], ConstantNullBlock[positions=1]]}]<>]]
```

The following join node:

```
InlineJoin[LEFT,[n{r}#4],[n{r}#4]]
|_Eval[[null[NULL] AS n#4]]
| \_EsRelation[employees][<no-fields>{r$}#7]
\_LocalRelation[[c{r}#2, n{r}#4],Page{blocks=[LongVectorBlock[vector=ConstantLongVector[positions=1, value=100]], ConstantNullBlock[positions=1]]}]
```

should NOT have `PruneLeftJoinOnNullMatchingField` applied, because the
right side is a `LocalRelation` (the `Aggregate` was optimized into a
`LocalRelation` containing the pre-computed aggregation results).
Pruning this join when the join key is null would discard the valid
aggregation results stored in the `LocalRelation`, incorrectly producing
null values instead of the expected count.

## Solution The fix ensures that `PruneLeftJoinOnNullMatchingField` only
applies to `LOOKUP JOIN` nodes, where `join.right()` is an `EsRelation`.
For `INLINE STATS` joins, the right side can be:

 - `Aggregate` (before optimization), or
 - `LocalRelation` (after the aggregate is optimized)

By checking `join.right() instanceof EsRelation`, we correctly skip the
pruning optimization for `INLINE STATS` joins, preserving the expected
query results when grouping by null.