### Elasticsearch Version

main@b6747b48ba0901a530ba51375e3576fdb8df8527

### Installed Plugins

_No response_

### Java Version

openjdk version "17.0.8" 2023-07-18

### OS Version

Ubuntu 23.04

### Problem Description

`row a = 1 | stats max(1) by a` (or some other agg) results in:
```
Caused by: java.lang.NullPointerException
	at java.base/java.util.Objects.requireNonNull(Objects.java:233)
	at java.base/java.util.ImmutableCollections$List12.<init>(ImmutableCollections.java:556)
	at java.base/java.util.List.of(List.java:812)
	at org.elasticsearch.xpack.esql.planner.AbstractPhysicalOperationProviders.aggregatesToFactory(AbstractPhysicalOperationProviders.java:185)
	at org.elasticsearch.xpack.esql.planner.AbstractPhysicalOperationProviders.groupingPhysicalOperation(AbstractPhysicalOperationProviders.java:133)
	at org.elasticsearch.xpack.esql.planner.LocalExecutionPlanner.planAggregation(LocalExecutionPlanner.java:207)
	at org.elasticsearch.xpack.esql.planner.LocalExecutionPlanner.plan(LocalExecutionPlanner.java:158)
	at org.elasticsearch.xpack.esql.planner.LocalExecutionPlanner.planAggregation(LocalExecutionPlanner.java:207)
	at org.elasticsearch.xpack.esql.planner.LocalExecutionPlanner.plan(LocalExecutionPlanner.java:158)
	at org.elasticsearch.xpack.esql.planner.LocalExecutionPlanner.planLimit(LocalExecutionPlanner.java:520)
	at org.elasticsearch.xpack.esql.planner.LocalExecutionPlanner.plan(LocalExecutionPlanner.java:176)
	at org.elasticsearch.xpack.esql.planner.LocalExecutionPlanner.planOutput(LocalExecutionPlanner.java:259)
	at org.elasticsearch.xpack.esql.planner.LocalExecutionPlanner.plan(LocalExecutionPlanner.java:198)
	at org.elasticsearch.xpack.esql.planner.LocalExecutionPlanner.plan(LocalExecutionPlanner.java:147)
	at org.elasticsearch.xpack.esql.plugin.ComputeService.runCompute(ComputeService.java:204)
	... 147 more
```

@nik9000 's thoughts on this: `I think this is just a kind of special case of "I can't EVAL in STATS". But it's a case the parser accepts.`

### Steps to Reproduce

Run the ESQL query from above.

### Logs (if relevant)

_No response_
x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/aggregate/CountDistinct.java
TypeResolution resolution = EsqlTypeResolutions.isExact(field(), sourceText(), DEFAULT);
        if (resolution.unresolved() || precision == null) {
            return resolution;
        }
        return isInteger(precision, sourceText(), SECOND);
    }
    @Override
    public AggregatorFunctionSupplier supplier(BigArrays bigArrays, List<Integer> inputChannels) {
        DataType type = field().dataType();
        int precision = this.precision == null ? DEFAULT_PRECISION : ((Number) this.precision.fold()).intValue();
        if (type == DataTypes.BOOLEAN) {
            
            return new CountDistinctBooleanAggregatorFunctionSupplier(bigArrays, inputChannels);
        }
        if (type == DataTypes.DATETIME || type == DataTypes.LONG) {
            return new CountDistinctLongAggregatorFunctionSupplier(bigArrays, inputChannels, precision);
        }
        if (type == DataTypes.INTEGER) {
            return new CountDistinctIntAggregatorFunctionSupplier(bigArrays, inputChannels, precision);
        }
        if (type == DataTypes.DOUBLE) {
            return new CountDistinctDoubleAggregatorFunctionSupplier(bigArrays, inputChannels, precision);
        }
        if (type == DataTypes.KEYWORD || type == DataTypes.IP) {
            return new CountDistinctBytesRefAggregatorFunctionSupplier(bigArrays, inputChannels, precision);
        }
        throw EsqlIllegalArgumentException.illegalDataType(type);
    }
}