Don't pass NULL to functions that take string inputs

In PHP 8, if you have a data processor that's supposed to output a string, but the field is blank, you get this message in your log (once per record outputted, so a lot):

Deprecated function: strpos(): Passing null to parameter #1 ($haystack) of type string is deprecated in Civi\DataProcessor\FieldOutputHandler\AbstractSimpleFieldOutputHandler->formatField() (line 89 of /home/jon/local/arteffect/web/sites/all/civicrm-custom/extensions/dataprocessor/Civi/DataProcessor/FieldOutputHandler/AbstractSimpleFieldOutputHandler.php) 

This fixes it.

Civi/DataProcessor/FieldOutputHandler/AbstractSimpleFieldOutputHandler.php

@@ -85,7 +85,7 @@ class AbstractSimpleFieldOutputHandler extends AbstractFieldOutputHandler {
    * @return \Civi\DataProcessor\FieldOutputHandler\FieldOutput
    */
   public function formatField($rawRecord, $formattedRecord) {
-    $formattedValue = $rawRecord[$this->inputFieldSpec->alias];
+    $formattedValue = $rawRecord[$this->inputFieldSpec->alias] ?? '';
     if (strpos($formattedValue, \CRM_Core_DAO::VALUE_SEPARATOR) !== false) {
       $formattedValue = explode(\CRM_Core_DAO::VALUE_SEPARATOR, substr($formattedValue, 1, -1));
       $formattedValue = implode(",", $formattedValue);