ExponentiateHelper: fix internal error and internal warning

[janedbal]

Probably fixes even https://github.com/phpstan/phpstan/issues/10125

Warning: A non-numeric value encountered on phar:///home/honza/Development/shipmonk/phpstan-rules/vendor/phpstan/phpstan/phpstan.phar/src/Type/ExponentiateHelper.php:72

Found in https://github.com/shipmonk-rnd/phpstan-rules tests when run with new https://github.com/phpstan/phpstan-src/commit/204ab2787f88bc31c4b2d30db0715997f3da4549