Faster UnionType->isNull()

[staabm]

Type->isNull() is invoked in TypeSpecifier->createForExpr on most expressions.

its shows up in a blackfire profile of https://github.com/phpstan/phpstan-src/blob/380b5ba72fc6bd35fb4f8c27a306a16ea42f24c1/tests/bench/data/big-constant-int-union.php :

before this PR:

➜  phpstan-src git:(2.2.x) ✗ php bin/phpstan analyze big-constant-int-union.php --debug -vv
Note: Using configuration file /Users/staabm/workspace/phpstan-src/phpstan.neon.dist.
Result cache not used because of debug mode.
/Users/staabm/workspace/phpstan-src/big-constant-int-union.php
Result cache was not saved because only files were passed as analysed paths.

                                                                                                                        
 [OK] No errors                                                                                                         
                                                                                                                        

Elapsed time: 0.42 seconds
Used memory: 58.5 MB

after this PR:

➜  phpstan-src git:(isnull) ✗ php bin/phpstan analyze big-constant-int-union.php --debug -vv
Note: Using configuration file /Users/staabm/workspace/phpstan-src/phpstan.neon.dist.
Result cache not used because of debug mode.
/Users/staabm/workspace/phpstan-src/big-constant-int-union.php
Result cache was not saved because only files were passed as analysed paths.

                                                                                                                        
 [OK] No errors                                                                                                         
                                                                                                                        

Elapsed time: 0.38 seconds
Used memory: 58.5 MB

refs phpstan/phpstan#15088

[phpstan-bot]

This pull request has been marked as ready for review.