[CodingStyle] Deprecate NullableCompareToNullRector

[TomasVotruba]

Deprecates NullableCompareToNullRector.

The rule turns a truthy if check on a nullable object into an explicit !== null compare:

 /** @var stdClass|null $value */
-if ($value) {
+if ($value !== null) {
 }

-if (! $value) {
+if ($value === null) {
 }

The result is still a weak, ambiguous comparison. !== null only tells us "not null" — it says nothing about what the value actually is, and it silently keeps working when the type later widens to a union of more classes.

An instanceof check is stronger and self-documenting:

if ($value instanceof stdClass) {
}

That is what the instanceof rule set already provides, so this rule only competes with it while producing the weaker output.

Rule is removed from the coding style level set and now throws on use, in line with the other recent deprecations.