[ESQL] Remove unused NullEquals class

[not-napoleon]

Although the ES|QL language doesn't support a null equals operator, we had a placeholder class for it with no @Evaluator methods defined. I presume this was to match the QL BinaryComparisonOperator enum, although I wasn't involved in the initial discussions around that choice. This PR removes the unused code, and associated references to it.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[not-napoleon]

These test cases were just copied over from the QL version of the optimizer rules.

[alex-spies]

LGTM, thanks @not-napoleon !

[elasticsearchmachine]

💔 Backport failed

Status	Branch	Result
❌	8.14	Commit could not be cherrypicked due to conflicts

You can use sqren/backport to manually backport by running backport --upstream elastic/elasticsearch --pr 108022

[not-napoleon]

The backport conflicts are related to #105407 (review) which wasn't backported. I don't think this is actually all that important to backport, so I'm just going to leave it as 8.15 only.

[astefan]

LGTM

To give a bit of background on what NullEquals is:

• it was introduced in ES SQL as part of SQL: implement null safe equality operator <=> #35871
• it returns true / false on equality against null values. Because SQL has a 3 value logic, any equality with null would result in null. This operator helps having a truly null equality

In ESQL we have the IS NULL / IS NOT NULL predicates that do the same thing. ES SQL also has these two predicates, but offered a shortcut in the form of an operator (NullEquals <=>).

ESQL also allows field == null BUT it's requiring a specific casting ie field == null::integer and this equality returns null. In a sense, field == null it's the ESQL variant of ES SQL's field = null.

[costin]

The operator is useful not only when comparing a field against null but also fields against each other - fieldA <=> fieldB and always return true/false even when at least one value is null.