Fix/nullable templates

[SplotyCode]

Fixed phpstan/phpstan#13048. Essentially, T of (A|null) subtracted by A is T of mixed

[ondrejmirtes]

1. This fixes a bunch of other issues, please check https://github.com/phpstan/phpstan-src/actions/runs/17309059596 and add regression tests for them too.
2. This has to be modified

   phpstan-src/src/Rules/Generics/TemplateTypeCheck.php

   Lines 119 to 142 in 07a29ab

   	if (
   	$boundTypeClass !== MixedType::class
   	&& $boundTypeClass !== ConstantArrayType::class
   	&& $boundTypeClass !== ArrayType::class
   	&& $boundTypeClass !== ConstantStringType::class
   	&& $boundTypeClass !== StringType::class
   	&& $boundTypeClass !== ConstantIntegerType::class
   	&& $boundTypeClass !== IntegerType::class
   	&& $boundTypeClass !== FloatType::class
   	&& $boundTypeClass !== BooleanType::class
   	&& $boundTypeClass !== ObjectWithoutClassType::class
   	&& $boundTypeClass !== ObjectType::class
   	&& $boundTypeClass !== ObjectShapeType::class
   	&& $boundTypeClass !== GenericObjectType::class
   	&& $boundTypeClass !== KeyOfType::class
   	&& $boundTypeClass !== IterableType::class
   	&& !$boundType instanceof UnionType
   	&& !$boundType instanceof IntersectionType
   	&& !$boundType instanceof TemplateType
   	) {
   	$messages[] = RuleErrorBuilder::message(sprintf($notSupportedBoundMessage, $templateTagName, $boundType->describe(VerbosityLevel::typeOnly())))
   	->identifier('generics.notSupportedBound')
   	->build();
   	}

   along with a test because of null is now supported.

[SplotyCode]

This also fixes phpstan/phpstan#12894 and phpstan/phpstan#12989

[phpstan-bot]

This pull request has been marked as ready for review.

[ondrejmirtes]

This also fixes phpstan/phpstan#12894 and phpstan/phpstan#12989

This text has to be in the main PR description in order to auto-close the issues. I'll do it manually for this one.

[ondrejmirtes]

Thank you.