lib/search/word_search: assert no missing rubys

lib/search/word_search/regrouping.dart

@@ -133,7 +133,7 @@ GroupedWordResult _regroup_words({
   required List<Map<String, Object?>> readingElementInfos,
   required List<Map<String, Object?>> readingElementRestrictions,
 }) {
-  final List<WordSearchRuby> result = [];
+  final List<WordSearchRuby> rubys = [];
 
   final kanjiElements_ =
       kanjiElements.where((element) => element['entryId'] == entryId).toList();
@@ -151,7 +151,7 @@ GroupedWordResult _regroup_words({
       final ruby = WordSearchRuby(
         base: readingElement['reading'] as String,
       );
-      result.add(ruby);
+      rubys.add(ruby);
 
       continue;
     }
@@ -173,12 +173,17 @@ GroupedWordResult _regroup_words({
         base: kanji,
         furigana: reading,
       );
-      result.add(ruby);
+      rubys.add(ruby);
     }
   }
 
+  assert(
+    rubys.isNotEmpty,
+    'No readings found for entryId: $entryId',
+  );
+
   return GroupedWordResult(
-    rubys: result,
+    rubys: rubys,
     readingInfos: Map.fromEntries(
       readingElementInfos.map((e) => MapEntry(
             e['reading'] as String,