**Build scan:**
https://gradle-enterprise.elastic.co/s/fvg2knbi424p4/tests/:x-pack:plugin:esql:test/org.elasticsearch.xpack.esql.expression.function.scalar.conditional.LeastTests/testEvaluateInManyThreads%20%7BTestCase=(%3Ckeyword%3E,%20%3Ckeyword%3E,%20%3Ckeyword%3E,%20%3Ckeyword%3E,%20%3Ckeyword%3E,%20%3Ckeyword%3E,%20%3Ckeyword%3E)%20null%20in%204%7D
**Reproduction lines:**
```
gradlew ':x-pack:plugin:esql:test' --tests "org.elasticsearch.xpack.esql.expression.function.scalar.conditional.GreatestTests.testEvaluateInManyThreads {TestCase=(<mv_integer>) null in 0}" -Dtests.seed=BCCD8A2AFCEA37CB -Dtests.locale=zh -Dtests.timezone=Europe/Guernsey -Druntime.java=21

gradlew ':x-pack:plugin:esql:test' --tests "org.elasticsearch.xpack.esql.expression.function.scalar.conditional.LeastTests.testEvaluateInManyThreads {TestCase=(<keyword>, <keyword>, <keyword>, <keyword>, <keyword>, <keyword>, <keyword>) null in 4}" -Dtests.seed=BCCD8A2AFCEA37CB -Dtests.locale=lt -Dtests.timezone=Indian/Christmas -Druntime.java=21

 gradlew ':x-pack:plugin:esql:test' --tests "org.elasticsearch.xpack.esql.expression.function.scalar.conditional.GreatestTests.testEvaluateInManyThreads {TestCase=(<mv_integer>) null in 0}" -Dtests.seed=BCCD8A2AFCEA37CB -Dtests.locale=zh -Dtests.timezone=Europe/Guernsey -Druntime.java=21
```

**Applicable branches:**
main

**Reproduces locally?:**
Yes

**Failure history:**
https://gradle-enterprise.elastic.co/scans/tests?tests.container=org.elasticsearch.xpack.esql.expression.function.scalar.conditional.LeastTests&tests.test=testEvaluateInManyThreads%20%7BTestCase%3D(%3Ckeyword%3E,%20%3Ckeyword%3E,%20%3Ckeyword%3E,%20%3Ckeyword%3E,%20%3Ckeyword%3E,%20%3Ckeyword%3E,%20%3Ckeyword%3E)%20null%20in%204%7D


**Failure excerpt:**
```
org.elasticsearch.xpack.esql.expression.function.scalar.conditional.GreatestTests > classMethod FAILED
    java.lang.AssertionError: expected:<[boolean, double, integer, ip, keyword, long, text, version]> but was:<[integer]>


```
x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/expression/function/AbstractFunctionTestCase.java
expression = new FoldNull().rule(expression);
        assertThat(expression.dataType(), equalTo(testCase.expectedType));
        assertTrue(expression.foldable());
        assertThat(expression.fold(), testCase.getMatcher());
        if (testCase.getExpectedWarnings() != null) {
            assertWarnings(testCase.getExpectedWarnings());
        }
    }
    public void testSerializationOfSimple() {
        assumeTrue("All test data types must be representable in order to build fields", testCase.allTypesAreRepresentable());
        assertSerialization(buildFieldExpression(testCase));
    }
    @AfterClass
    public static void testFunctionInfo() {
        FunctionDefinition definition = definition();
        if (definition == null) {
            LogManager.getLogger(getTestClass()).info("Skipping function info checks because the function isn't registered");
            return;
        }
        EsqlFunctionRegistry.FunctionDescription description = EsqlFunctionRegistry.description(definition);
        List<EsqlFunctionRegistry.ArgSignature> args = description.args();
        List<Set<String>> typesFromSignature = new ArrayList<>();
        Set<String> returnFromSignature = new HashSet<>();
        for (int i = 0; i < args.size(); i++) {
            typesFromSignature.add(new HashSet<>());
        }
        for (Map.Entry<List<DataType>, DataType> entry : signatures.entrySet()) {
            List<DataType> types = entry.getKey();
            for (int i = 0; i < args.size() && i < types.size(); i++) {
                typesFromSignature.get(i).add(types.get(i).esType());