Issue #3304404 by Michelle: Null $value causing PHP 8 deprecation warning

modules/salesforce_mapping/src/SalesforceMappingFieldPluginBase.php

@@ -242,7 +242,7 @@ abstract class SalesforceMappingFieldPluginBase extends PluginBase implements Sa
         break;
     }
 
-    if ($field_definition['length'] > 0 && strlen($value) > $field_definition['length']) {
+    if (!empty($value) && $field_definition['length'] > 0 && strlen($value) > $field_definition['length']) {
       $value = substr($value, 0, $field_definition['length']);
     }