Fix equality of empty lists ({} = {} -> true) (#1721)

[c-schuler]

Problem

{} = {} returned null instead of true (and {} != {} returned null instead of false). Per the spec, the = operator for lists returns true when the lists "have the same elements by value, in the same order" — two empty lists are vacuously equal.

Root cause

EqualEvaluator.listsEqual had an over-broad guard that returned null whenever either list was empty:

if (!leftIterator.hasNext() || !rightIterator.hasNext()) {
    return null
}

This conflated "empty list" with "null". It was introduced in the Equal/Equivalent refactor (#1668). The existing loop/tail logic already handles empty lists correctly, so the guard was both unnecessary and wrong.

Fix

Removed the guard. Behavior is now:

expression	result
{} = {}	true
{} != {}	false
{null} = {null}	true (nulls considered equal
{} = null / null = {}	null (null operand)

Null-element handling is unchanged

The removed guard only fired for empty lists (zero elements); lists that contain nulls have hasNext() == true, so they were never affected. The existing element-wise logic continues to implement the spec's two clauses correctly - a null matched against another null is equal, a null against a non-null (or a trailing null on a length mismatch) yields null:

expression	result
{1, null} = {1, null}	true
{1, null} = {1, 2}	null
{1} = {1, null}	null
{1} = {1, 2}	false

EquivalentEvaluator.listEquivalent has no analogous guard, so this bug was isolated to EqualEvaluator.

Engine test corrections

The engine's own tests asserted the buggy null for {} = {}; updated to match the conformance suite (EqualEmptyListAndEmptyList -> true, NotEqualEmptyAndEmpty -> false):

• ListOperatorsTest.kt: EqualEmptyListAndEmptyList, NotEqualEmptyAndEmpty.
• CqlInternalTypeRepresentationSuiteTest.kt: equal({}, EMPTY_LIST) (parameterized over timezones).

Validation

• Full :engine:jvmTest: 228 tests, 0 failures.
• CQL Tests conformance: EqualEmptyListAndEmptyList and NotEqualEmptyAndEmpty now pass; no regressions.

Fixes #1721

[github-actions]

Related Issues

The following open issues may be related to this PR:

Issue	Title	Score	Matched Terms
#1723	Distinct operation on a list with all nulls does not return a list but a single null	26.5	"null expression", "list null", "return null", "null null", "null instead", operator, instead, returns, return, null, expression, nulls, returned, tests, operators (path), org (path), cql (path), list (path), test (path)
#1794	Point from Interval[null, null] is returning null instead of erroring	24	"per spec", "null expression", "null null", "null instead", result, true, instead, return, bug, spec, null, expression, per, tests, operators (path), org (path), cql (path), test (path)
#1133	ToList is not implemented according to the spec	22.5	"operand null", operator, returns, lists, bug, operand, behavior, element, already, spec, null, logic, expression, value, type (path), java (path), src (path), opencds (path), engine (path), main (path), org (path), cql (path), cqf (path), elm (path), execution (path), list (path), test (path)
#1735	Expression Tuple equivalence on tuples with mismatched number of items does not return false	21.5	"cql tests", "null false", "returns true", operator, true, returns, return, both, false, null, expression, equivalent, tests, value, org (path), cql (path), test (path)
#1763	CqlComparisonOperatorsTest "groupName": "Unit Comparison" have many failures	21	"cql tests", "null true", "false null", true, returns, bug, false, equality, failures, null, expression, either, tests, engine (path), org (path), cql (path), test (path)

Tip: If this PR addresses any of these issues, please link them using Closes #NNN or Refs #NNN in the PR description.

[github-actions]

Formatting check succeeded!

[c-schuler]

Closing this as the cql-tests test is found to be in error: cqframework/cql-tests#129