Don't iterate over empty string of field mappings.

modules/salesforce_mapping/src/Entity/SalesforceMapping.php

@@ -477,11 +477,13 @@ class SalesforceMapping extends ConfigEntityBase implements SalesforceMappingInt
   public function getFieldMappings() {
     // @TODO #fieldMappingField
     $fields = [];
-    foreach ($this->field_mappings as $i => $field) {
-      $fields[$i] = $this->fieldManager()->createInstance(
-         $field['drupal_field_type'],
-         $field + ['mapping' => $this]
-       );
+    if (!empty($this->field_mappings)) {
+      foreach ($this->field_mappings as $i => $field) {
+        $fields[$i] = $this->fieldManager()->createInstance(
+          $field['drupal_field_type'],
+          $field + ['mapping' => $this]
+        );
+      }
     }
     return $fields;
   }