Refactor MvContains null handling by reusing existing IsNullEvaluatorFactory and adding global constant boolean evaluators for improved reusability.

Author: mjmbischoff

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/expression/function/scalar/multivalue/MvContainsTests.java

@@ -342,8 +342,8 @@ protected static List<TestCaseSupplier> anyNullIsNull(
                                     return new TestCaseSupplier.TestCase(
                                         typeDataWithNull,
                                         nullPosition == 0
-                                            ? "MvContainsNullSupersetEvaluator[subsetField=Attribute[channel=1]]"
-                                            : "ConstantBooleanTrueEvaluator",
+                                            ? "IsNullEvaluator[field=Attribute[channel=1]]"
+                                            : "ConstantTrue",
                                         expectedType.expectedType(nullPosition, DataType.BOOLEAN, originalTestCase),
                                         equalTo(nullPosition == 1)
                                     );