This shouldn't be affecting 5.5 for what I've tested. We have tests for soft deleting feature (e.g. DatabaseSoftDeletingScopeTest and DatabaseSoftDeletingTraitTest) and the Continuous Integration builds for PHP 7.2 aren't failing: https://travis-ci.org/laravel/framework/builds
Note that this same line of SoftDeletingScope has already been fixed: https://github.com/laravel/framework/blob/5.5/src/Illuminate/Database/Eloquent/SoftDeletingScope.php#L55 Going back in time we can see that this was fixed in 5.4 after merging PR #20258. 😉
I solved by changing "$originalWhereCount = count($query->wheres);" for "$originalWhereCount = is_null($query->wheres)
? 0 : count($query->wheres);" in the function "callScope(callable $scope, $parameters = )" in the file "/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Builder.php"