Prevent passing null to explode

modules/salesforce_mapping/src/SalesforceMappingFieldPluginBase.php

@@ -295,7 +295,10 @@ abstract class SalesforceMappingFieldPluginBase extends PluginBase implements Sa
         break;
 
       case 'multipicklist':
-        if (!is_array($value)) {
+        if (empty($value)) {
+          $value = [];
+        }
+        if (is_string($value)) {
           $value = explode(';', $value);
           $value = array_map('trim', $value);
         }