Migration\Handler\SetValue not compatible with PHP 8.1

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### Preconditions

1. Magento 1 with tier prices
2. Migration to Magento 2.4.5 (opensource to opensource)
3. PHP 8.1

### Steps to reproduce

1. Migrate Magento 1 to Magento 2

### Expected result

1. Migration completes successfully

### Actual result

Error in the OrderGrids Step:

```
[2022-12-05T08:22:49.790153+00:00][DEBUG][mode: data][stage: data migration][step: Tier Price Step][table: catalog_product_entity_tier_price]: migrating
0% [>---------------------------] Remaining Time: < 1 sec
In ErrorHandler.php line 62:

[Exception]
Deprecated Functionality: substr(): Passing null to parameter #1 ($string)
of type string is deprecated in /var/www/html/magento/vendor/magento/data-m
igration-tool/src/Migration/Handler/SetValue.php on line 35

Exception trace:
at /var/www/html/magento/vendor/magento/framework/App/ErrorHandler.php:62
Magento\Framework\App\ErrorHandler->handler() at n/a:n/a
substr() at /var/www/html/magento/vendor/magento/data-migration-tool/src/Migration/Handler/SetValue.php:35
Migration\Handler\SetValue->handle() at /var/www/html/magento/vendor/magento/data-migration-tool/src/Migration/RecordTransformer.php:126
Migration\RecordTransformer->applyHandlers() at /var/www/html/magento/vendor/magento/data-migration-tool/src/Migration/RecordTransformer.php:74
Migration\RecordTransformer->transform() at /var/www/html/magento/vendor/magento/data-migration-tool/src/Migration/Step/TierPrice/Data.php:139
Migration\Step\TierPrice\Data->perform() at /var/www/html/magento/vendor/magento/data-migration-tool/src/Migration/Mode/AbstractMode.php:82
Migration\Mode\AbstractMode->runStage() at /var/www/html/magento/vendor/magento/data-migration-tool/src/Migration/Mode/Data.php:124
Migration\Mode\Data->runData() at /var/www/html/magento/vendor/magento/data-migration-tool/src/Migration/Mode/Data.php:69
Migration\Mode\Data->run() at /var/www/html/magento/vendor/magento/data-migration-tool/src/Migration/Console/MigrateDataCommand.php:59
Migration\Console\MigrateDataCommand->execute() at /var/www/html/magento/vendor/symfony/console/Command/Command.php:255
Symfony\Component\Console\Command\Command->run() at /var/www/html/magento/vendor/magento/framework/Interception/Interceptor.php:58
Migration\Console\MigrateDataCommand\Interceptor->___callParent() at /var/www/html/magento/vendor/magento/framework/Interception/Interceptor.php:138
Migration\Console\MigrateDataCommand\Interceptor->Magento\Framework\Interception\{closure}() at /var/www/html/magento/vendor/magento/framework/Interception/Interceptor.php:153
Migration\Console\MigrateDataCommand\Interceptor->___callPlugins() at /var/www/html/magento/generated/code/Migration/Console/MigrateDataCommand/Interceptor.php:77
Migration\Console\MigrateDataCommand\Interceptor->run() at /var/www/html/magento/vendor/symfony/console/Application.php:1021
Symfony\Component\Console\Application->doRunCommand() at /var/www/html/magento/vendor/symfony/console/Application.php:275
Symfony\Component\Console\Application->doRun() at /var/www/html/magento/vendor/magento/framework/Console/Cli.php:116
Magento\Framework\Console\Cli->doRun() at /var/www/html/magento/vendor/symfony/console/Application.php:149
Symfony\Component\Console\Application->run() at /var/www/html/magento/bin/magento:23
```

### Additional notes

The problem is triggered by the `map-tier-price.xml.dist` configuration

```xml

catalog_product_entity_group_price.value_id





catalog_product_entity_tier_price.value_id




```

because the `Migration\Handler\SetValue` class throws an error if NULL should be set:

```php
value = (strtoupper($value) === 'NULL') ? null : $value;
}

/**
* @inheritdoc
*/
public function handle(Record $recordToHandle, Record $oppositeRecord)
{
$this->validate($recordToHandle);
$valueStored = $recordToHandle->getValue($this->field);
$operator = substr($this->value, 0, 1);
$value = substr($this->value, 1);
switch ($operator) {
case '+':
$value = $valueStored + $value;
break;
case '-';
$value = $valueStored - $value;
break;
default:
$value = $this->value;
}
$recordToHandle->setValue($this->field, $value);
}
}
```

The problem are the lines

```php
$operator = substr($this->value, 0, 1);
$value = substr($this->value, 1);
```

if `$this->value` is null, since null musn't be used anymore in `substr`.

As a workaround I changed the lines to

```php
$operator = substr($this->value ?? '', 0, 1);
$value = substr($this->value ?? '', 1);
```

and could finish the migration.

Start with src/Migration/Handler/SetValue.php, especially handle() and the substr() calls, then review the map-tier-price.xml.dist configuration that passes NULL. Reproduce the Magento 1 to Magento 2.4.5 migration with PHP 8.1 and verify that the Tier Price step completes without the deprecation error.

Written by the indexing model from the issue text.