Try to fix word iterator notice, a=chris

diff --git a/src/library/index_bundle_iterators/WordIterator.php b/src/library/index_bundle_iterators/WordIterator.php
index da7a54ef6..8029e918b 100644
--- a/src/library/index_bundle_iterators/WordIterator.php
+++ b/src/library/index_bundle_iterators/WordIterator.php
@@ -991,8 +991,8 @@ class WordIterator extends IndexBundleIterator
             $partition_info = $this->dictionary_info[$this->generation_pointer];
             $this->current_generation = $partition_info['PARTITION'];
             $postings = $this->getGenerationPostings($this->generation_pointer);
-            $this->current_doc_offset = ($postings) ?
-                $postings[$this->current_offset]['DOC_MAP_INDEX'] : -1;
+            $this->current_doc_offset =
+                $postings[$this->current_offset]['DOC_MAP_INDEX'] ?? -1;
         }
         return [$this->current_generation, $this->current_doc_offset];
     }